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[libvirt] [PATCHv4 01/17] util: add VIR_(APPEND|INSERT|DELETE)_ELEMENT



I noticed when writing the backend functions for virNetworkUpdate that
I was repeating the same sequence of memmove, VIR_REALLOC, nXXX-- (and
messed up the args to memmove at least once), and had seen the same
sequence in a lot of other places, so I decided to write a few
utility functions/macros - see the .h file for full documentation.

The intent is to reduce the number of lines of code, but more
importantly to eliminate the need to check the element size and
element count arithmetic every time we need to do this (I *always*
make at least one mistake.)

VIR_INSERT_ELEMENT: insert one element at an arbitrary index within an
  array of objects. The size of each object is determined
  automatically by the macro using sizeof(*array). If a pointer to a
  new element is provided, its contents are copied into the inserted
  space then the original contents are 0'ed out; if no newelem is
  provided the new space is set to all 0. Compile-time assignment and size
  compatibility between the array and the new element is guaranteed
  (see explanation below [*])

VIR_INSERT_ELEMENT_COPY: identical to VIR_INSERT_ELEMENT, except that
  the original contents of newelem are not cleared to 0 (i.e. a copy
  is made).

VIR_APPEND_ELEMENT: This is just a special case of VIR_INSERT_ELEMENT
  that "inserts" one past the current last element.

VIR_APPEND_ELEMENT_COPY: identical to VIR_APPEND_ELEMENT, except that
  the original contents of newelem are not cleared to 0 (i.e. a copy
  is made).

VIR_DELETE_ELEMENT: delete one element at an arbitrary index within an
  array of objects. It's assumed that the element being deleted is
  already saved elsewhere (or cleared, if that's what is appropriate).

All five of these macros have an _INPLACE variant, which skips the
memory re-allocation of the array, assuming that the caller has
already done it (when inserting) or will do it later (when deleting).

Note that VIR_DELETE_ELEMENT* can return a failure, but only if an
invalid index is given (index + amount to delete is > current array
size), so in most cases you can safely ignore the return (that's why
the helper function virDeleteElementsN isn't declared with
ATTRIBUTE_RETURN_CHECK).

[*] One initial problem with the INSERT and APPEND macros was that,
due to both the array pointer and newelem pointer being cast to void*
when passing to virInsertElementsN(), any chance of type-checking was
lost. If we were going to move in newelem with a memmove anyway, we
would be no worse off for this. However, most current open-coded
insert/append operations use direct struct assignment to move the new
element into place (or just populate the new element directly) - thus
use of the new macros would open a possibility for new usage errors
that didn't exist before (e.g. accidentally sending &newelemptr rather
than newelemptr - I actually did this quite a lot in my test
conversions of existing code).

But thanks to Eric Blake's clever thinking, I was able to modify the
INSERT and APPEND macros so that they *do* check for both assignment
and size compatibility of *ptr (an element in the array) and newelem
(the element being copied into the new position of the array). This is
done via clever use of the C89-guaranteed fact that the sizeof()
operator is must have *no* side effects (so an assignment inside
sizeof() is checked for validity, but not actually evaluated), and the
fact that virInsertElementsN has a "# of new elements" argument that we want
to always be 1.

What we do is replace the "1" in that argument with a call to
VIR_TYPEMATCH(ptr, newelem), which returns 1 on success, and generates
a compile error on failure. VIR_TYPEMATCH does three things:

   * sizeof(*(a) = *(b)) assures that *a and *b are
     assignment-compatible (they may still have a different size
     though! e.g. longVar = intVar) (If not, there is a compile-time
     error. If so, the result of that subexpression is sizeof(*(a)),
     i.e. one element of the array)

   * sizeof(*(a) = *(b)) == sizeof(*(b)) checks if *a and *b are also
     of the same size (so that, e.g. you don't accidentally copy an
     int plus the random bytes following it into an array of long). It
     evaluates to 1 if they are the same, and 0 otherwise.

   * sizeof(char[2 * (result of previous step) - 1]) evaluates to 1 if
     the previous step was successful (char [(2*1) - 1] ==> char[1]),
     of generates a compile error if it wasn't successful
     (char[2*0) -1] ==> char[-1], which isn't legal).

So we end up sending "1" to the caller, and in the meantime check that we've
actually added the correct &'s and/or *'s to the arguments. (Whew!)

The result is that we've been relieved of the burden of getting the
math right for the arguments to memmove when expanding/contracting the
array, and haven't lost the type checking of ptr and newelem.
---
 cfg.mk                   |   2 +-
 src/libvirt_private.syms |   2 +
 src/util/memory.c        | 106 +++++++++++++++++++++++++++++-
 src/util/memory.h        | 167 ++++++++++++++++++++++++++++++++++++++++++++++-
 4 files changed, 274 insertions(+), 3 deletions(-)

diff --git a/cfg.mk b/cfg.mk
index c4ae195..f218eb6 100644
--- a/cfg.mk
+++ b/cfg.mk
@@ -314,7 +314,7 @@ sc_flags_usage:
 
 # Avoid functions that should only be called via macro counterparts.
 sc_prohibit_internal_functions:
-	@prohibit='vir(Free|AllocN?|ReallocN|File(Close|Fclose|Fdopen)) *\(' \
+	@prohibit='vir(Free|AllocN?|ReallocN|(Insert|Delete)ElementsN|File(Close|Fclose|Fdopen)) *\(' \
 	halt='use VIR_ macros instead of internal functions'		\
 	  $(_sc_search_regexp)
 
diff --git a/src/libvirt_private.syms b/src/libvirt_private.syms
index 499ab3b..ad7dc17 100644
--- a/src/libvirt_private.syms
+++ b/src/libvirt_private.syms
@@ -819,8 +819,10 @@ virLogUnlock;
 virAlloc;
 virAllocN;
 virAllocVar;
+virDeleteElementsN;
 virExpandN;
 virFree;
+virInsertElementsN;
 virReallocN;
 virResizeN;
 virShrinkN;
diff --git a/src/util/memory.c b/src/util/memory.c
index 0f7aca1..40d612d 100644
--- a/src/util/memory.c
+++ b/src/util/memory.c
@@ -1,7 +1,7 @@
 /*
  * memory.c: safer memory allocation
  *
- * Copyright (C) 2010-2011 Red Hat, Inc.
+ * Copyright (C) 2010-2012 Red Hat, Inc.
  * Copyright (C) 2008 Daniel P. Berrange
  *
  * This library is free software; you can redistribute it and/or
@@ -253,6 +253,110 @@ void virShrinkN(void *ptrptr, size_t size, size_t *countptr, size_t toremove)
     }
 }
 
+/**
+ * virInsertElementsN:
+ * @ptrptr:   pointer to hold address of allocated memory
+ * @size:     the size of one element in bytes
+ * @at:       index within array where new elements should be added
+ * @countptr: variable tracking number of elements currently allocated
+ * @add:      number of elements to add
+ * @newelems: pointer to array of one or more new elements to move into
+ *            place (the originals will be zeroed out if successful
+ *            and if clearOriginal is true)
+ * @clearOriginal: false if the new item in the array should be copied
+ *            from the original, and the original left intact.
+ *            true if the original should be 0'd out on success.
+ * @inPlace:  false if we should expand the allocated memory before
+ *            moving, true if we should assume someone else *has
+ *            already* done that.
+ *
+ * Re-allocate an array of 'count' elements, each sizeof(*ptr) bytes
+ * long, to be 'count' + 'add' elements long, then appropriately move
+ * the elements starting at ptr[at] up by 'count' elements, copy the
+ * items from 'newelems' into ptr[at], then store the address of
+ * allocated memory in 'ptr' and the new size in 'count'.  If
+ * 'newelems' is NULL, the new elements at ptr[at] are instead filled
+ * with zero.
+ *
+ * Returns -1 on failure, 0 on success
+ */
+int
+virInsertElementsN(void *ptrptr, size_t size, size_t at,
+                   size_t *countptr,
+                   size_t add, void *newelems,
+                   bool clearOriginal, bool inPlace)
+{
+    if (at > *countptr)
+       return -1;
+
+    if (inPlace) {
+        *countptr += add;
+    } else if (virExpandN(ptrptr, size, countptr, add) < 0) {
+        return -1;
+    }
+
+    /* memory was successfully re-allocated. Move up all elements from
+     * ptrptr[at] to the end (if we're not "inserting" at the end
+     * already), memcpy in the new elements, and clear the elements
+     * from their original location. Remember that *countptr has
+     * already been updated with new element count!
+     */
+    if (at < *countptr - add) {
+        memmove(*(char**)ptrptr + (size * (at + add)),
+                *(char**)ptrptr + (size * at),
+                size * (*countptr - add - at));
+    }
+
+    if (newelems) {
+        memcpy(*(char**)ptrptr + (size * at), newelems, size * add);
+        if (clearOriginal)
+           memset((char*)newelems, 0, size * add);
+    } else if (inPlace || (at < *countptr - add)) {
+        /* NB: if inPlace, even memory at the end wasn't initialized */
+        memset(*(char**)ptrptr + (size * at), 0, size * add);
+    }
+
+    return 0;
+}
+
+/**
+ * virDeleteElementsN:
+ * @ptr:     pointer to hold address of allocated memory
+ * @size:    the size of one element in bytes
+ * @at:      index within array where new elements should be deleted
+ * @count:   variable tracking number of elements currently allocated
+ * @remove:  number of elements to remove
+ * @inPlace: false if we should shrink the allocated memory when done,
+ *           true if we should assume someone else will do that.
+ *
+ * Re-allocate an array of 'count' elements, each sizeof(*ptr)
+ * bytes long, to be 'count' - 'remove' elements long, then store the
+ * address of allocated memory in 'ptr' and the new size in 'count'.
+ * If 'count' <= 'remove', the entire array is freed.
+ *
+ * Returns -1 on failure, 0 on success
+ */
+int
+virDeleteElementsN(void *ptrptr, size_t size, size_t at,
+                   size_t *countptr, size_t remove,
+                   bool inPlace)
+{
+    if (at + remove > *countptr)
+        return -1;
+
+    /* First move down the elements at the end that won't be deleted,
+     * then realloc. We assume that the items being deleted have
+     * already been cleared.
+    */
+    memmove(*(char**)ptrptr + (size * at),
+            *(char**)ptrptr + (size * (at + remove)),
+            size * (*countptr - remove - at));
+    if (inPlace)
+        *countptr -= remove;
+    else
+        virShrinkN(ptrptr, size, countptr, remove);
+    return 0;
+}
 
 /**
  * Vir_Alloc_Var:
diff --git a/src/util/memory.h b/src/util/memory.h
index ad8ee64..5a1c4df 100644
--- a/src/util/memory.h
+++ b/src/util/memory.h
@@ -1,7 +1,7 @@
 /*
  * memory.c: safer memory allocation
  *
- * Copyright (C) 2010-2011 Red Hat, Inc.
+ * Copyright (C) 2010-2012 Red Hat, Inc.
  * Copyright (C) 2008 Daniel P. Berrange
  *
  * This library is free software; you can redistribute it and/or
@@ -59,6 +59,13 @@ int virResizeN(void *ptrptr, size_t size, size_t *alloc, size_t count,
     ATTRIBUTE_RETURN_CHECK ATTRIBUTE_NONNULL(1) ATTRIBUTE_NONNULL(3);
 void virShrinkN(void *ptrptr, size_t size, size_t *count, size_t toremove)
     ATTRIBUTE_NONNULL(1) ATTRIBUTE_NONNULL(3);
+int virInsertElementsN(void *ptrptr, size_t size, size_t at, size_t *countptr,
+                       size_t add, void *newelem,
+                       bool clearOriginal, bool inPlace)
+    ATTRIBUTE_RETURN_CHECK ATTRIBUTE_NONNULL(1) ATTRIBUTE_NONNULL(4);
+int virDeleteElementsN(void *ptrptr, size_t size, size_t at, size_t *countptr,
+                       size_t remove, bool inPlace)
+    ATTRIBUTE_NONNULL(1) ATTRIBUTE_NONNULL(4);
 int virAllocVar(void *ptrptr,
                 size_t struct_size,
                 size_t element_size,
@@ -159,6 +166,164 @@ void virFree(void *ptrptr) ATTRIBUTE_NONNULL(1);
     virShrinkN(&(ptr), sizeof(*(ptr)), &(count), remove)
 
 /*
+ * VIR_TYPEMATCH:
+ *
+ * The following macro seems a bit cryptic, so it needs a thorough
+ * explanation. Its purpose is to check for assignment compatibility
+ * and identical size between two values without creating any side
+ * effects (by doing something silly like actually assigning one to
+ * the other). Note that it takes advantage of the C89-guaranteed
+ * property of sizeof() - it cannot have any side effects, so anything
+ * that happens inside sizeof() will not have any effect at runtime.
+ *
+ * VIR_TYPEMATCH evaluates to "1" if the two passed values are both
+ * assignment-compatible and the same size, and otherwise generates a
+ * compile-time error. It determines the result by performing the
+ * following three operations:
+ *
+ *    * sizeof(*(a) = *(b)) assures that *a and *b are
+ *      assignment-compatible (they may still have a different size
+ *      though! e.g. longVar = intVar) (If not, there is a compile-time
+ *      error. If so, the result of that subexpression is sizeof(*(a)),
+ *      i.e. one element of the array)
+ *
+ *    * sizeof(*(a) = *(b)) == sizeof(*(b)) checks if *a and *b are also
+ *      of the same size (so that, e.g. you don't accidentally copy an
+ *      int plus the random bytes following it into an array of long). It
+ *      evaluates to 1 if they are the same, and 0 otherwise.
+ *
+ *    * sizeof(char[2 * (result of previous step) - 1]) evaluates to 1
+ *      if the previous step was successful (char [(2*1) - 1] == *
+ *      char[1]), of generates a compile error if it wasn't successful
+ *      (char[2*0) -1] == * char[-1], which isn't legal and again
+ *      generates a compile error).
+ *
+ * So VIR_TYPECHECK(a, b) will either abort the compile with an error,
+ * or evaluate to "1", and in the meantime check that we've actually
+ * added the correct &'s and/or *'s to the arguments. (Whew!)
+*/
+# define VIR_TYPEMATCH(a, b) \
+    sizeof(char[2 * (sizeof(*(a) = *(b)) == sizeof(*(b))) - 1])
+
+/**
+ * VIR_INSERT_ELEMENT:
+ * @ptr:     pointer to array of objects (*not* ptr to ptr)
+ * @at:      index within array where new elements should be added
+ * @count:   variable tracking number of elements currently allocated
+ * @newelem: the new element to move into place (*not* a pointer to
+ *           the element, but the element itself).
+ *           (the original will be zeroed out if successful)
+ *
+ * Re-allocate an array of 'count' elements, each sizeof(*ptr) bytes
+ * long, to be 'count' + 1 elements long, then appropriately move
+ * the elements starting at ptr[at] up by 1 element, copy the
+ * item 'newelem' into ptr[at], then store the address of
+ * allocated memory in 'ptr' and the new size in 'count'.
+ *
+ * VIR_INSERT_ELEMENT_COPY is identical, but doesn't clear out the
+ *   original element to 0 on success, so there are two copies of the
+ *   element. This is useful if the "element" is actually just a
+ *   pointer to the real data, and you want to maintain a reference to
+ *   it for use after the insert is completed; but if the "element" is
+ *   an object that points to other allocated memory, having multiple
+ *   copies can cause problems (e.g. double free).
+ *
+ * VIR_INSERT_ELEMENT_*INPLACE are identical, but assume any necessary
+ *   memory re-allocation has already been done.
+ *
+ * VIR_INSERT_ELEMENT_* all need to send "1" as the "add" argument to
+ * virInsertElementsN (which has the currently-unused capability of
+ * inserting multiple items at once). We use this to our advantage by
+ * replacing it with VIR_TYPECHECK(ptr, &newelem) so that we can be
+ * assured ptr and &newelem are of compatible types.
+ *
+ * Returns -1 on failure, 0 on success
+ *
+ *
+ */
+# define VIR_INSERT_ELEMENT(ptr, at, count, newelem) \
+    virInsertElementsN(&(ptr), sizeof(*(ptr)), at, &(count),    \
+                       VIR_TYPEMATCH(ptr, &(newelem)), &(newelem), true, false)
+# define VIR_INSERT_ELEMENT_COPY(ptr, at, count, newelem) \
+    virInsertElementsN(&(ptr), sizeof(*(ptr)), at, &(count), \
+                       VIR_TYPEMATCH(ptr, &(newelem)), &(newelem), false, false)
+# define VIR_INSERT_ELEMENT_INPLACE(ptr, at, count, newelem) \
+    virInsertElementsN(&(ptr), sizeof(*(ptr)), at, &(count), \
+                       VIR_TYPEMATCH(ptr, &(newelem)), &(newelem), true, true)
+# define VIR_INSERT_ELEMENT_COPY_INPLACE(ptr, at, count, newelem) \
+    virInsertElementsN(&(ptr), sizeof(*(ptr)), at, &(count), \
+                       VIR_TYPEMATCH(ptr, &(newelem)), &(newelem), false, true)
+
+/**
+ * VIR_APPEND_ELEMENT:
+ * @ptr:     pointer to array of objects (*not* ptr to ptr)
+ * @count:   variable tracking number of elements currently allocated
+ * @newelem: the new element to move into place (*not* a pointer to
+ *           the element, but the element itself).
+ *           (the original will be zeroed out if successful)
+ *
+ * Re-allocate an array of 'count' elements, each sizeof(*ptr) bytes
+ * long, to be 'count' + 1 elements long, then copy the item from
+ * 'newelem' into ptr[count+1], and store the address of allocated
+ * memory in 'ptr' and the new size in 'count'. If 'newelem' is NULL,
+ * the new element at ptr[at] is instead filled with zero.
+ *
+ * VIR_APPEND_ELEMENT_COPY is identical, but doesn't clear out the
+ *   original element to 0 on success, so there are two copies of the
+ *   element. This is useful if the "element" is actually just a
+ *   pointer to the real data, and you want to maintain a reference to
+ *   it for use after the append is completed; but if the "element" is
+ *   an object that points to other allocated memory, having multiple
+ *   copies can cause problems (e.g. double free).
+ *
+ * VIR_APPEND_ELEMENT_*INPLACE are identical, but assume any
+ *   necessary memory re-allocation has already been done.
+ *
+ * VIR_APPEND_ELEMENT_* all need to send "1" as the "add" argument to
+ * virInsertElementsN (which has the currently-unused capability of
+ * inserting multiple items at once). We use this to our advantage by
+ * replacing it with VIR_TYPECHECK(ptr, &newelem) so that we can be
+ * assured ptr and &newelem are of compatible types.
+ *
+ * Returns -1 on failure, 0 on success
+ *
+ *
+ */
+# define VIR_APPEND_ELEMENT(ptr, count, newelem) \
+    virInsertElementsN(&(ptr), sizeof(*(ptr)), count, &(count),  \
+                       VIR_TYPEMATCH(ptr, &(newelem)), &(newelem), true, false)
+# define VIR_APPEND_ELEMENT_COPY(ptr, count, newelem) \
+    virInsertElementsN(&(ptr), sizeof(*(ptr)), count, &(count),  \
+                       VIR_TYPEMATCH(ptr, &(newelem)), &(newelem), false, false)
+# define VIR_APPEND_ELEMENT_INPLACE(ptr, count, newelem) \
+    virInsertElementsN(&(ptr), sizeof(*(ptr)), count, &(count),  \
+                       VIR_TYPEMATCH(ptr, &(newelem)), &(newelem), true, true)
+# define VIR_APPEND_ELEMENT_COPY_INPLACE(ptr, count, newelem) \
+    virInsertElementsN(&(ptr), sizeof(*(ptr)), count, &(count),  \
+                       VIR_TYPEMATCH(ptr, &(newelem)), &(newelem), false, true)
+
+/**
+ * VIR_DELETE_ELEMENT:
+ * @ptr:   pointer to array of objects (*not* ptr to ptr)
+ * @at:    index within array where new elements should be deleted
+ * @count: variable tracking number of elements currently allocated
+ *
+ * Re-allocate an array of 'count' elements, each sizeof(*ptr)
+ * bytes long, to be 'count' - 1 elements long, then store the
+ * address of allocated memory in 'ptr' and the new size in 'count'.
+ * If 'count' <= 1, the entire array is freed.
+ *
+ * VIR_DELETE_ELEMENT_INPLACE is identical, but assumes any
+ *   necessary memory re-allocation will be done later.
+ *
+ * Returns -1 on failure, 0 on success
+ */
+# define VIR_DELETE_ELEMENT(ptr, at, count) \
+    virDeleteElementsN(&(ptr), sizeof(*(ptr)), at, &(count), 1, false)
+# define VIR_DELETE_ELEMENT_INPLACE(ptr, at, count) \
+    virDeleteElementsN(&(ptr), sizeof(*(ptr)), at, &(count), 1, true)
+
+/*
  * VIR_ALLOC_VAR_OVERSIZED:
  * @M: size of base structure
  * @N: number of array elements in trailing array
-- 
1.7.11.7


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