Replace use of sys_siglist[] with strsignal().

This commit back-patches the v12-era commits a73d08319, cc92cca43,
and 7570df0f3 into supported pre-v12 branches.  The net effect is to
eliminate our former dependency on the never-standard sys_siglist[]
array, instead using POSIX-standard strsignal(3).

What motivates doing this now is that glibc just removed sys_siglist[]
from the set of symbols available to newly-built programs.  While our
code can survive without sys_siglist[], it then fails to print any
description of the signal that killed a child process, which is a
non-negligible loss of friendliness.  We can expect that people will
be wanting to build the back branches on platforms that include this
change, so we need to do something.

Since strsignal(3) has existed for quite a long time, and we've not
had any trouble with these patches so far in v12, it seems safe to
back-patch into older branches.

Discussion: https://postgr.es/m/3179114.1594853308@sss.pgh.pa.us

1 files changed, 64 insertions, 0 deletions

diff --git a/src/port/pgstrsignal.c b/src/port/pgstrsignal.c
new file mode 100644
index 00000000000..7724edf56e1
--- /dev/null
+++ b/src/port/pgstrsignal.c
@@ -0,0 +1,64 @@
+/*-------------------------------------------------------------------------
+ *
+ * pgstrsignal.c
+ *	  Identify a Unix signal number
+ *
+ * On platforms compliant with modern POSIX, this just wraps strsignal(3).
+ * Elsewhere, we do the best we can.
+ *
+ * This file is not currently built in MSVC builds, since it's useless
+ * on non-Unix platforms.
+ *
+ * Portions Copyright (c) 1996-2018, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ * IDENTIFICATION
+ *	  src/port/pgstrsignal.c
+ *
+ *-------------------------------------------------------------------------
+ */
+
+#include "c.h"
+
+
+/*
+ * pg_strsignal
+ *
+ * Return a string identifying the given Unix signal number.
+ *
+ * The result is declared "const char *" because callers should not
+ * modify the string.  Note, however, that POSIX does not promise that
+ * the string will remain valid across later calls to strsignal().
+ *
+ * This version guarantees to return a non-NULL pointer, although
+ * some platforms' versions of strsignal() reputedly do not.
+ *
+ * Note that the fallback cases just return constant strings such as
+ * "unrecognized signal".  Project style is for callers to print the
+ * numeric signal value along with the result of this function, so
+ * there's no need to work harder than that.
+ */
+const char *
+pg_strsignal(int signum)
+{
+	const char *result;
+
+	/*
+	 * If we have strsignal(3), use that --- but check its result for NULL.
+	 */
+#ifdef HAVE_STRSIGNAL
+	result = strsignal(signum);
+	if (result == NULL)
+		result = "unrecognized signal";
+#else
+
+	/*
+	 * We used to have code here to try to use sys_siglist[] if available.
+	 * However, it seems that all platforms with sys_siglist[] have also had
+	 * strsignal() for many years now, so that was just a waste of code.
+	 */
+	result = "(signal names not available on this platform)";
+#endif
+
+	return result;
+}