Improve pqexpbuffer.c to use modern vsnprintf implementations efficiently.

When using a C99-compliant vsnprintf, we can use its report of the required
buffer size to avoid making multiple loops through the formatting logic.
This is similar to the changes recently made in stringinfo.c, but we can't
use psprintf.c here because in libpq we don't want to exit() on error.
(The behavior pqexpbuffer.c has historically used is to mark the
PQExpBuffer as "broken", ie empty, if it runs into any fatal problem.)

To avoid duplicating code more than necessary, I refactored
printfPQExpBuffer and appendPQExpBuffer to share a subroutine that's
very similar to psprintf.c's pvsnprintf in spirit.

Author: tglsfdc

src/interfaces/libpq/pqexpbuffer.c

@@ -8,7 +8,8 @@
  *
  * This module is essentially the same as the backend's StringInfo data type,
  * but it is intended for use in frontend libpq and client applications.
- * Thus, it does not rely on palloc() nor elog().
+ * Thus, it does not rely on palloc() nor elog(), nor psprintf.c which
+ * will exit() on error.
  *
  * It does rely on vsnprintf(); if configure finds that libc doesn't provide
  * a usable vsnprintf(), then a copy of our own implementation of it will
@@ -36,6 +37,10 @@
 /* All "broken" PQExpBuffers point to this string. */
 static const char oom_buffer[1] = "";
 
+static bool
+appendPQExpBufferVA(PQExpBuffer str, const char *fmt, va_list args)
+__attribute__((format(PG_PRINTF_ATTRIBUTE, 2, 0)));
+
 
 /*
  * markPQExpBufferBroken
@@ -231,45 +236,20 @@ void
 printfPQExpBuffer(PQExpBuffer str, const char *fmt,...)
 {
 	va_list		args;
-	size_t		avail;
-	int			nprinted;
+	bool		done;
 
 	resetPQExpBuffer(str);
 
 	if (PQExpBufferBroken(str))
 		return;					/* already failed */
 
-	for (;;)
+	/* Loop in case we have to retry after enlarging the buffer. */
+	do
 	{
-		/*
-		 * Try to format the given string into the available space; but if
-		 * there's hardly any space, don't bother trying, just fall through to
-		 * enlarge the buffer first.
-		 */
-		if (str->maxlen > str->len + 16)
-		{
-			avail = str->maxlen - str->len - 1;
-			va_start(args, fmt);
-			nprinted = vsnprintf(str->data + str->len, avail,
-								 fmt, args);
-			va_end(args);
-
-			/*
-			 * Note: some versions of vsnprintf return the number of chars
-			 * actually stored, but at least one returns -1 on failure. Be
-			 * conservative about believing whether the print worked.
-			 */
-			if (nprinted >= 0 && nprinted < (int) avail - 1)
-			{
-				/* Success.  Note nprinted does not include trailing null. */
-				str->len += nprinted;
-				break;
-			}
-		}
-		/* Double the buffer size and try again. */
-		if (!enlargePQExpBuffer(str, str->maxlen))
-			return;				/* oops, out of memory */
-	}
+		va_start(args, fmt);
+		done = appendPQExpBufferVA(str, fmt, args);
+		va_end(args);
+	} while (!done);
 }
 
 /*
@@ -284,43 +264,118 @@ void
 appendPQExpBuffer(PQExpBuffer str, const char *fmt,...)
 {
 	va_list		args;
-	size_t		avail;
-	int			nprinted;
+	bool		done;
 
 	if (PQExpBufferBroken(str))
 		return;					/* already failed */
 
-	for (;;)
+	/* Loop in case we have to retry after enlarging the buffer. */
+	do
+	{
+		va_start(args, fmt);
+		done = appendPQExpBufferVA(str, fmt, args);
+		va_end(args);
+	} while (!done);
+}
+
+/*
+ * appendPQExpBufferVA
+ * Shared guts of printfPQExpBuffer/appendPQExpBuffer.
+ * Attempt to format data and append it to str.  Returns true if done
+ * (either successful or hard failure), false if need to retry.
+ */
+static bool
+appendPQExpBufferVA(PQExpBuffer str, const char *fmt, va_list args)
+{
+	size_t		avail;
+	size_t		needed;
+	int			nprinted;
+
+	/*
+	 * Try to format the given string into the available space; but if there's
+	 * hardly any space, don't bother trying, just enlarge the buffer first.
+	 */
+	if (str->maxlen > str->len + 16)
 	{
 		/*
-		 * Try to format the given string into the available space; but if
-		 * there's hardly any space, don't bother trying, just fall through to
-		 * enlarge the buffer first.
+		 * Note: we intentionally leave one byte unused, as a guard against
+		 * old broken versions of vsnprintf.
 		 */
-		if (str->maxlen > str->len + 16)
+		avail = str->maxlen - str->len - 1;
+
+		errno = 0;
+
+		nprinted = vsnprintf(str->data + str->len, avail, fmt, args);
+
+		/*
+		 * If vsnprintf reports an error other than ENOMEM, fail.
+		 */
+		if (nprinted < 0 && errno != 0 && errno != ENOMEM)
 		{
-			avail = str->maxlen - str->len - 1;
-			va_start(args, fmt);
-			nprinted = vsnprintf(str->data + str->len, avail,
-								 fmt, args);
-			va_end(args);
+			markPQExpBufferBroken(str);
+			return true;
+		}
 
+		/*
+		 * Note: some versions of vsnprintf return the number of chars
+		 * actually stored, not the total space needed as C99 specifies.  And
+		 * at least one returns -1 on failure.	Be conservative about
+		 * believing whether the print worked.
+		 */
+		if (nprinted >= 0 && (size_t) nprinted < avail - 1)
+		{
+			/* Success.  Note nprinted does not include trailing null. */
+			str->len += nprinted;
+			return true;
+		}
+
+		if (nprinted >= 0 && (size_t) nprinted > avail)
+		{
 			/*
-			 * Note: some versions of vsnprintf return the number of chars
-			 * actually stored, but at least one returns -1 on failure. Be
-			 * conservative about believing whether the print worked.
+			 * This appears to be a C99-compliant vsnprintf, so believe its
+			 * estimate of the required space. (If it's wrong, the logic will
+			 * still work, but we may loop multiple times.)  Note that the
+			 * space needed should be only nprinted+1 bytes, but we'd better
+			 * allocate one more than that so that the test above will succeed
+			 * next time.
+			 *
+			 * In the corner case where the required space just barely
+			 * overflows, fail.
 			 */
-			if (nprinted >= 0 && nprinted < (int) avail - 1)
+			if (nprinted > INT_MAX - 2)
 			{
-				/* Success.  Note nprinted does not include trailing null. */
-				str->len += nprinted;
-				break;
+				markPQExpBufferBroken(str);
+				return true;
 			}
+			needed = nprinted + 2;
 		}
-		/* Double the buffer size and try again. */
-		if (!enlargePQExpBuffer(str, str->maxlen))
-			return;				/* oops, out of memory */
+		else
+		{
+			/*
+			 * Buffer overrun, and we don't know how much space is needed.
+			 * Estimate twice the previous buffer size, but not more than
+			 * INT_MAX.
+			 */
+			if (avail >= INT_MAX / 2)
+				needed = INT_MAX;
+			else
+				needed = avail * 2;
+		}
+	}
+	else
+	{
+		/*
+		 * We have to guess at how much to enlarge, since we're skipping the
+		 * formatting work.
+		 */
+		needed = 32;
 	}
+
+	/* Increase the buffer size and try again. */
+	if (!enlargePQExpBuffer(str, needed))
+		return true;			/* oops, out of memory */
+
+	return false;
 }
 
 /*