Adjust the behavior of the PQExpBuffer code to make it have well-defined

results (ie, an empty "broken" buffer) if memory overrun occurs anywhere
along the way to filling the buffer.  The previous coding would just silently
discard portions of the intended buffer contents, as exhibited in trouble
report from Sam Mason.  Also, tweak psql's main loop to correctly detect
and report such overruns.  There's probably much more that should be done
in this line, but this is a start.

src/interfaces/libpq/pqexpbuffer.c

@@ -17,7 +17,7 @@
  * Portions Copyright (c) 1996-2008, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/interfaces/libpq/pqexpbuffer.c,v 1.24 2008/01/01 19:46:00 momjian Exp $
+ * $PostgreSQL: pgsql/src/interfaces/libpq/pqexpbuffer.c,v 1.25 2008/11/26 00:26:23 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -32,6 +32,32 @@
 #include "win32.h"
 #endif
 
+
+/* All "broken" PQExpBuffers point to this string. */
+static const char oom_buffer[1] = "";
+
+
+/*
+ * markPQExpBufferBroken
+ *
+ * Put a PQExpBuffer in "broken" state if it isn't already.
+ */
+static void
+markPQExpBufferBroken(PQExpBuffer str)
+{
+	if (str->data != oom_buffer)
+		free(str->data);
+	/*
+	 * Casting away const here is a bit ugly, but it seems preferable to
+	 * not marking oom_buffer const.  We want to do that to encourage the
+	 * compiler to put oom_buffer in read-only storage, so that anyone who
+	 * tries to scribble on a broken PQExpBuffer will get a failure.
+	 */
+	str->data = (char *) oom_buffer;
+	str->len = 0;
+	str->maxlen = 0;
+}
+
 /*
  * createPQExpBuffer
  *
@@ -61,6 +87,7 @@ initPQExpBuffer(PQExpBuffer str)
 	str->data = (char *) malloc(INITIAL_EXPBUFFER_SIZE);
 	if (str->data == NULL)
 	{
+		str->data = (char *) oom_buffer;		/* see comment above */
 		str->maxlen = 0;
 		str->len = 0;
 	}
@@ -96,12 +123,10 @@ destroyPQExpBuffer(PQExpBuffer str)
 void
 termPQExpBuffer(PQExpBuffer str)
 {
-	if (str->data)
-	{
+	if (str->data != oom_buffer)
 		free(str->data);
-		str->data = NULL;
-	}
 	/* just for luck, make the buffer validly empty. */
+	str->data = (char *) oom_buffer;		/* see comment above */
 	str->maxlen = 0;
 	str->len = 0;
 }
@@ -109,15 +134,24 @@ termPQExpBuffer(PQExpBuffer str)
 /*
  * resetPQExpBuffer
  *		Reset a PQExpBuffer to empty
+ *
+ * Note: if possible, a "broken" PQExpBuffer is returned to normal.
  */
 void
 resetPQExpBuffer(PQExpBuffer str)
 {
 	if (str)
 	{
-		str->len = 0;
-		if (str->data)
+		if (str->data != oom_buffer)
+		{
+			str->len = 0;
 			str->data[0] = '\0';
+		}
+		else
+		{
+			/* try to reinitialize to valid state */
+			initPQExpBuffer(str);
+		}
 	}
 }
 
@@ -126,7 +160,8 @@ resetPQExpBuffer(PQExpBuffer str)
  * Make sure there is enough space for 'needed' more bytes in the buffer
  * ('needed' does not include the terminating null).
  *
- * Returns 1 if OK, 0 if failed to enlarge buffer.
+ * Returns 1 if OK, 0 if failed to enlarge buffer.  (In the latter case
+ * the buffer is left in "broken" state.)
  */
 int
 enlargePQExpBuffer(PQExpBuffer str, size_t needed)
@@ -134,13 +169,19 @@ enlargePQExpBuffer(PQExpBuffer str, size_t needed)
 	size_t		newlen;
 	char	   *newdata;
 
+	if (PQExpBufferBroken(str))
+		return 0;				/* already failed */
+
 	/*
 	 * Guard against ridiculous "needed" values, which can occur if we're fed
 	 * bogus data.	Without this, we can get an overflow or infinite loop in
 	 * the following.
 	 */
 	if (needed >= ((size_t) INT_MAX - str->len))
+	{
+		markPQExpBufferBroken(str);
 		return 0;
+	}
 
 	needed += str->len + 1;		/* total space required now */
 
@@ -173,6 +214,8 @@ enlargePQExpBuffer(PQExpBuffer str, size_t needed)
 		str->maxlen = newlen;
 		return 1;
 	}
+
+	markPQExpBufferBroken(str);
 	return 0;
 }
 
@@ -192,6 +235,9 @@ printfPQExpBuffer(PQExpBuffer str, const char *fmt,...)
 
 	resetPQExpBuffer(str);
 
+	if (PQExpBufferBroken(str))
+		return;					/* already failed */
+
 	for (;;)
 	{
 		/*
@@ -240,6 +286,9 @@ appendPQExpBuffer(PQExpBuffer str, const char *fmt,...)
 	size_t		avail;
 	int			nprinted;
 
+	if (PQExpBufferBroken(str))
+		return;					/* already failed */
+
 	for (;;)
 	{
 		/*