Add more efficient functions to pqformat API.

There's three prongs to achieve greater efficiency here:

1) Allow reusing a stringbuffer across pq_beginmessage/endmessage,
   with the new pq_beginmessage_reuse/endmessage_reuse. This can be
   beneficial both because it avoids allocating the initial buffer,
   and because it's more likely to already have an correctly sized
   buffer.

2) Replacing pq_sendint() with pq_sendint$width() inline
   functions. Previously unnecessary and unpredictable branches in
   pq_sendint() were needed. Additionally the replacement functions
   are implemented more efficiently.  pq_sendint is now deprecated, a
   separate commit will convert all in-tree callers.

3) Add pq_writeint$width(), pq_writestring(). These rely on sufficient
   space in the StringInfo's buffer, avoiding individual space checks
   & potential individual resizing.  To allow this to be used for
   strings, expose mbutil.c's MAX_CONVERSION_GROWTH.

Followup commits will make use of these facilities.

Author: Andres Freund
Discussion: https://postgr.es/m/[email protected]

1 files changed, 11 insertions, 0 deletions

diff --git a/src/include/mb/pg_wchar.h b/src/include/mb/pg_wchar.h
index d57ef017cb4..9227d634f66 100644
--- a/src/include/mb/pg_wchar.h
+++ b/src/include/mb/pg_wchar.h
@@ -305,6 +305,17 @@ typedef enum pg_enc
 #define PG_VALID_FE_ENCODING(_enc)	PG_VALID_ENCODING(_enc)
 
 /*
+ * When converting strings between different encodings, we assume that space
+ * for converted result is 4-to-1 growth in the worst case. The rate for
+ * currently supported encoding pairs are within 3 (SJIS JIS X0201 half width
+ * kanna -> UTF8 is the worst case).  So "4" should be enough for the moment.
+ *
+ * Note that this is not the same as the maximum character width in any
+ * particular encoding.
+ */
+#define MAX_CONVERSION_GROWTH  4
+
+/*
  * Table for mapping an encoding number to official encoding name and
  * possibly other subsidiary data.  Be careful to check encoding number
  * before accessing a table entry!