buf: support peeking at string contents

Right now, the only way to get at the contents of a virBuffer is
to destroy it.  But there are cases in my upcoming patches where
peeking at the contents makes life easier.  I suppose this does
open up the potential for bad code to dereference a stale pointer,
by disregarding the docs that the return value is invalid on the
next virBuf operation, but such is life.

* src/util/buf.h (virBufferCurrentContent): New declaration.
* src/util/buf.c (virBufferCurrentContent): Implement it.
* src/libvirt_private.syms (buf.h): Export it.
* tests/virbuftest.c (testBufAutoIndent): Test it.

src/libvirt_private.syms

@@ -20,6 +20,7 @@ virBufferAddChar;
 virBufferAdjustIndent;
 virBufferAsprintf;
 virBufferContentAndReset;
+virBufferCurrentContent;
 virBufferError;
 virBufferEscape;
 virBufferEscapeSexpr;

src/util/buf.c

@@ -174,13 +174,33 @@ virBufferAddChar(virBufferPtr buf, char c)
     virBufferAdd(buf, &c, 1);
 }
 
+/**
+ * virBufferCurrentContent:
+ * @buf: Buffer
+ *
+ * Get the current content from the buffer.  The content is only valid
+ * until the next operation on @buf, and an empty string is returned if
+ * no content is present yet.
+ *
+ * Returns the buffer content or NULL in case of error.
+ */
+const char *
+virBufferCurrentContent(virBufferPtr buf)
+{
+    if (!buf || buf->error)
+        return NULL;
+    return buf->use ? buf->content : "";
+}
+
 /**
  * virBufferContentAndReset:
  * @buf: Buffer
  *
  * Get the content from the buffer and free (only) the buffer structure.
  * The caller owns the returned string & should free it when no longer
- * required. The buffer object is reset to its initial state.
+ * required. The buffer object is reset to its initial state.  This
+ * interface intentionally returns NULL instead of an empty string if
+ * there is no content.
  *
  * Returns the buffer content or NULL in case of error.
  */

src/util/buf.h

@@ -37,6 +37,7 @@ struct _virBuffer {
 };
 # endif
 
+const char *virBufferCurrentContent(virBufferPtr buf);
 char *virBufferContentAndReset(virBufferPtr buf);
 void virBufferFreeAndReset(virBufferPtr buf);
 int virBufferError(const virBufferPtr buf);

tests/virbuftest.c

@@ -73,6 +73,10 @@ static int testBufAutoIndent(const void *data ATTRIBUTE_UNUSED)
         ret = -1;
     }
     virBufferAdjustIndent(buf, 3);
+    if (STRNEQ(virBufferCurrentContent(buf), "")) {
+        TEST_ERROR("Wrong content");
+        ret = -1;
+    }
     if (virBufferGetIndent(buf, false) != 3 ||
         virBufferGetIndent(buf, true) != 3 ||
         virBufferError(buf)) {
@@ -102,6 +106,10 @@ static int testBufAutoIndent(const void *data ATTRIBUTE_UNUSED)
     }
     virBufferAdjustIndent(buf, 2);
     virBufferAddLit(buf, "1");
+    if (STRNEQ(virBufferCurrentContent(buf), "  1")) {
+        TEST_ERROR("Wrong content");
+        ret = -1;
+    }
     if (virBufferGetIndent(buf, false) != 2 ||
         virBufferGetIndent(buf, true) != 0) {
         TEST_ERROR("Wrong indentation");