Remove XLogFileNameP() from the tree

XLogFileNameP() is a wrapper routine able to build a palloc'd string for
a WAL segment name, which is used for error string generation.  There
were several code paths where it gets called in a critical section,
where memory allocation is not allowed.  This results in triggering
an assertion failure instead of generating the wanted error message.

Another, more annoying, problem is that if the allocation to generate
the WAL segment name fails on OOM, then the failure would be escalated
to a PANIC.

This removes the routine and all its callers are replaced with a logic
using a fixed-size buffer.  This way, all the existing mistakes are
fixed and future ones are prevented.

Author: Masahiko Sawada
Reviewed-by: Michael Paquier, Álvaro Herrera
Discussion: https://postgr.es/m/CA+fd4k5gC9H4uoWMLg9K_QfNrnkkdEw+-AFveob9YX7z8JnKTA@mail.gmail.com

src/include/access/xlog.h

@@ -288,7 +288,6 @@ extern bool RecoveryIsPaused(void);
 extern void SetRecoveryPause(bool recoveryPause);
 extern TimestampTz GetLatestXTime(void);
 extern TimestampTz GetCurrentChunkReplayStartTime(void);
-extern char *XLogFileNameP(TimeLineID tli, XLogSegNo segno);
 
 extern void UpdateControlFile(void);
 extern uint64 GetSystemIdentifier(void);

src/include/access/xlog_internal.h

@@ -152,6 +152,10 @@ typedef XLogLongPageHeaderData *XLogLongPageHeader;
 /* Length of XLog file name */
 #define XLOG_FNAME_LEN	   24
 
+/*
+ * Generate a WAL segment file name.  Do not use this macro in a helper
+ * function allocating the result generated.
+ */
 #define XLogFileName(fname, tli, logSegNo, wal_segsz_bytes)	\
 	snprintf(fname, MAXFNAMELEN, "%08X%08X%08X", tli,		\
 			 (uint32) ((logSegNo) / XLogSegmentsPerXLogId(wal_segsz_bytes)), \