Use libc version as a collation version on glibc systems.

Using glibc's version string to detect potential collation definition
changes is not 100% reliable, but it's better than nothing.  Currently
this affects only collations explicitly provided by "libc".  More work
will be needed to handle the default collation.

Author: Thomas Munro, based on a suggestion from Christoph Berg
Reviewed-by: Peter Eisentraut
Discussion: https://postgr.es/m/4b76c6d4-ae5e-0dc6-7d0d-b5c796a07e34%402ndquadrant.com

doc/src/sgml/ref/alter_collation.sgml

@@ -129,6 +129,16 @@ HINT:  Rebuild all objects affected by this collation and run ALTER COLLATION pg
    does not actually check whether all affected objects have been rebuilt
    correctly.
   </para>
+  <para>
+   When using collations provided by <literal>libc</literal> and
+   <productname>PostgreSQL</productname> was built with the GNU C library, the
+   C library's version is used as a collation version.  Since collation
+   definitions typically change only with GNU C library releases, this provides
+   some defense against corruption, but it is not completely reliable.
+  </para>
+  <para>
+   Currently, there is no version tracking for the database default collation.
+  </para>
 
   <para>
    The following query can be used to identify all collations in the current

src/backend/utils/adt/pg_locale.c

@@ -70,6 +70,10 @@
 #include <unicode/ucnv.h>
 #endif
 
+#ifdef __GLIBC__
+#include <gnu/libc-version.h>
+#endif
+
 #ifdef WIN32
 /*
  * This Windows file defines StrNCpy. We don't need it here, so we undefine
@@ -1499,7 +1503,7 @@ pg_newlocale_from_collation(Oid collid)
 char *
 get_collation_actual_version(char collprovider, const char *collcollate)
 {
-	char	   *collversion;
+	char	   *collversion = NULL;
 
 #ifdef USE_ICU
 	if (collprovider == COLLPROVIDER_ICU)
@@ -1523,7 +1527,13 @@ get_collation_actual_version(char collprovider, const char *collcollate)
 	}
 	else
 #endif
-		collversion = NULL;
+	if (collprovider == COLLPROVIDER_LIBC)
+	{
+#if defined(__GLIBC__)
+		/* Use the glibc version because we don't have anything better. */
+		collversion = pstrdup(gnu_get_libc_version());
+#endif
+	}
 
 	return collversion;
 }

src/bin/pg_dump/t/002_pg_dump.pl

@@ -1376,8 +1376,8 @@ my %tests = (
 	'CREATE COLLATION test0 FROM "C"' => {
 		create_order => 76,
 		create_sql   => 'CREATE COLLATION test0 FROM "C";',
-		regexp       => qr/^
-		  \QCREATE COLLATION public.test0 (provider = libc, locale = 'C');\E/xm,
+		regexp       =>
+		  qr/CREATE COLLATION public.test0 \(provider = libc, locale = 'C'(, version = '[^']*')?\);/m,
 		collation => 1,
 		like      => { %full_runs, section_pre_data => 1, },
 	},