string: make VIR_STRDUP easier to use

While reviewing proposed VIR_STRDUP conversions, I've already noticed
several places that do:

if (str && VIR_STRDUP(dest, str) < 0)

which can be simplified by allowing str to be NULL (something that
strdup() doesn't allow).  Meanwhile, code that wants to ensure a
non-NULL dest regardless of the source can check for <= 0.

Also, make it part of the VIR_STRDUP contract that macro arguments
are evaluated exactly once.

* src/util/virstring.h (VIR_STRDUP, VIR_STRDUP_QUIET, VIR_STRNDUP)
(VIR_STRNDUP_QUIET): Improve contract.
* src/util/virstring.c (virStrdup, virStrndup): Change return
conventions.
* docs/hacking.html.in: Document this.
* HACKING: Regenerate.

Signed-off-by: Eric Blake <eblake@redhat.com>

Conflicts:
	HACKING
	docs/hacking.html.in

(cherry picked from commit 6b74a9f5d98e066f8dfdf5d5ccda68230b516246)

HACKING

@@ -234,6 +234,11 @@ But if negating a complex condition is too ugly, then at least add braces:
 
 Preprocessor
 ============
+Macros defined with an ALL_CAPS name should generally be assumed to be unsafe
+with regards to arguments with side-effects (that is, MAX(a++, b--) might
+increment a or decrement b too many or too few times). Exceptions to this rule
+are explicitly documented for macros in viralloc.h and virstring.h.
+
 For variadic macros, stick with C99 syntax:
 
   #define vshPrint(_ctl, ...) fprintf(stdout, __VA_ARGS__)
@@ -539,12 +544,12 @@ virStrncpy(dest, src, strlen(src), sizeof(dest)).
   VIR_STRNDUP(char *dst, const char *src, size_t n);
 
 You should avoid using strdup or strndup directly as they do not report
-out-of-memory error. Use VIR_STRDUP or VIR_STRNDUP macros instead. Note, that
+out-of-memory error, and do not allow a NULL source. Use VIR_STRDUP or
-these two behave similar to VIR_ALLOC: on success zero is returned, otherwise
+VIR_STRNDUP macros instead, which return 0 for NULL source, 1 for successful
-the result is -1 and dst is guaranteed to be NULL. In very specific cases,
+copy, and -1 for allocation failure with the error already reported. In very
-when you don't want to report the out-of-memory error, you can use
+specific cases, when you don't want to report the out-of-memory error, you can
-VIR_STRDUP_QUIET or VIR_STRNDUP_QUIET, but such usage is very rare and usually
+use VIR_STRDUP_QUIET or VIR_STRNDUP_QUIET, but such usage is very rare and
-considered a flaw.
+usually considered a flaw.
 
 
 Variable length string buffer

docs/hacking.html.in

@@ -303,7 +303,14 @@
   }
 </pre>
 
-    <h2><a href="types">Preprocessor</a></h2>
+    <h2><a name="preprocessor">Preprocessor</a></h2>
+
+    <p>Macros defined with an ALL_CAPS name should generally be
+      assumed to be unsafe with regards to arguments with side-effects
+      (that is, MAX(a++, b--) might increment a or decrement b too
+      many or too few times).  Exceptions to this rule are explicitly
+      documented for macros in viralloc.h and virstring.h.
+    </p>
 
     <p>
       For variadic macros, stick with C99 syntax:
@@ -647,9 +654,10 @@
 </pre>
     <p>
       You should avoid using strdup or strndup directly as they do not report
-      out-of-memory error. Use VIR_STRDUP or VIR_STRNDUP macros instead.  Note,
+      out-of-memory error, and do not allow a NULL source. Use
-      that these two behave similar to VIR_ALLOC: on success zero is returned,
+      VIR_STRDUP or VIR_STRNDUP macros instead, which return 0 for
-      otherwise the result is -1 and dst is guaranteed to be NULL. In very
+      NULL source, 1 for successful copy, and -1 for allocation
+      failure with the error already reported. In very
       specific cases, when you don't want to report the out-of-memory error, you
       can use VIR_STRDUP_QUIET or VIR_STRNDUP_QUIET, but such usage is very rare
       and usually considered a flaw.

src/util/virstring.c

@@ -194,7 +194,7 @@ size_t virStringListLength(char **strings)
  * caller's body where virStrdup is called from. Consider
  * using VIR_STRDUP which sets these automatically.
  *
- * Returns: 0 on success, -1 otherwise.
+ * Returns: 0 for NULL src, 1 on successful copy, -1 otherwise.
  */
 int
 virStrdup(char **dest,
@@ -205,13 +205,15 @@ virStrdup(char **dest,
           const char *funcname,
           size_t linenr)
 {
+    if (!src)
+        return 0;
     if (!(*dest = strdup(src))) {
         if (report)
             virReportOOMErrorFull(domcode, filename, funcname, linenr);
         return -1;
     }
 
-    return 0;
+    return 1;
 }
 
 /**
@@ -231,7 +233,7 @@ virStrdup(char **dest,
  * caller's body where virStrndup is called from. Consider
  * using VIR_STRNDUP which sets these automatically.
  *
- * Returns: 0 on success, -1 otherwise.
+ * Returns: 0 for NULL src, 1 on successful copy, -1 otherwise.
  */
 int
 virStrndup(char **dest,
@@ -243,11 +245,13 @@ virStrndup(char **dest,
            const char *funcname,
            size_t linenr)
 {
+    if (!src)
+        return 0;
     if (!(*dest = strndup(src, n))) {
         if (report)
             virReportOOMErrorFull(domcode, filename, funcname, linenr);
         return -1;
     }
 
-   return 0;
+   return 1;
 }

src/util/virstring.h

@@ -40,11 +40,11 @@ size_t virStringListLength(char **strings);
 /* Don't call these directly - use the macros below */
 int virStrdup(char **dest, const char *src, bool report, int domcode,
               const char *filename, const char *funcname, size_t linenr)
-    ATTRIBUTE_RETURN_CHECK ATTRIBUTE_NONNULL(1) ATTRIBUTE_NONNULL(2);
+    ATTRIBUTE_RETURN_CHECK ATTRIBUTE_NONNULL(1);
 
 int virStrndup(char **dest, const char *src, size_t n, bool report, int domcode,
                const char *filename, const char *funcname, size_t linenr)
-    ATTRIBUTE_RETURN_CHECK ATTRIBUTE_NONNULL(1) ATTRIBUTE_NONNULL(2);
+    ATTRIBUTE_RETURN_CHECK ATTRIBUTE_NONNULL(1);
 
 /**
  * VIR_STRDUP:
@@ -53,7 +53,10 @@ int virStrndup(char **dest, const char *src, size_t n, bool report, int domcode,
  *
  * Duplicate @src string and store it into @dst.
  *
- * Returns -1 on failure (with OOM error reported), 0 on success
+ * This macro is safe to use on arguments with side effects.
+ *
+ * Returns -1 on failure (with OOM error reported), 0 if @src was NULL,
+ * 1 if @src was copied
  */
 # define VIR_STRDUP(dst, src) virStrdup(&(dst), src, true, VIR_FROM_THIS, \
                                         __FILE__, __FUNCTION__, __LINE__)
@@ -65,7 +68,9 @@ int virStrndup(char **dest, const char *src, size_t n, bool report, int domcode,
  *
  * Duplicate @src string and store it into @dst.
  *
- * Returns -1 on failure, 0 on success
+ * This macro is safe to use on arguments with side effects.
+ *
+ * Returns -1 on failure, 0 if @src was NULL, 1 if @src was copied
  */
 # define VIR_STRDUP_QUIET(dst, src) virStrdup(&(dst), src, false, 0, NULL, NULL, 0)
 
@@ -78,7 +83,10 @@ int virStrndup(char **dest, const char *src, size_t n, bool report, int domcode,
  * Duplicate @src string and store it into @dst. If @src is longer than @n,
  * only @n bytes are copied and terminating null byte '\0' is added.
  *
- * Returns -1 on failure (with OOM error reported), 0 on success
+ * This macro is safe to use on arguments with side effects.
+ *
+ * Returns -1 on failure (with OOM error reported), 0 if @src was NULL,
+ * 1 if @src was copied
  */
 # define VIR_STRNDUP(dst, src, n) virStrndup(&(dst), src, n, true,    \
                                              VIR_FROM_THIS, __FILE__, \
@@ -93,7 +101,9 @@ int virStrndup(char **dest, const char *src, size_t n, bool report, int domcode,
  * Duplicate @src string and store it into @dst. If @src is longer than @n,
  * only @n bytes are copied and terminating null byte '\0' is added.
  *
- * Returns -1 on failure, 0 on success
+ * This macro is safe to use on arguments with side effects.
+ *
+ * Returns -1 on failure, 0 if @src was NULL, 1 if @src was copied
  */
 # define VIR_STRNDUP_QUIET(dst, src, n) virStrndup(&(dst), src, n, false, \
                                                    0, NULL, NULL, 0)