Use whitelist to choose files scanned with pg_verify_checksums

The original implementation of pg_verify_checksums used a blacklist to
decide which files should be skipped for scanning as they do not include
data checksums, like pg_internal.init or pg_control.  However, this
missed two things:
- Some files are created within builds of EXEC_BACKEND and these were
not listed, causing failures on Windows.
- Extensions may create custom files in data folders, causing the tool
to equally fail.

This commit switches to a whitelist-like method instead by checking if
the files to scan are authorized relation files.  This is close to a
reverse-engineering of what is defined in relpath.c in charge of
building the relation paths, and we could consider refactoring what this
patch does so as all routines are in a single place.  This is left for
later.

This is based on a suggestion from Andres Freund.  TAP tests are updated
so as multiple file patterns are tested.  The bug has been spotted by
various buildfarm members as a result of b34e84f which has introduced
the TAP tests of pg_verify_checksums.

Author: Michael Paquier
Reviewed-by: Andrew Dunstan, Michael Banck
Discussion: https://postgr.es/m/20181012005614.GC26424@paquier.xyz
Backpatch-through: 11

Author: michaelpq

src/bin/pg_verify_checksums/pg_verify_checksums.c

@@ -15,6 +15,7 @@
 
 #include "catalog/pg_control.h"
 #include "common/controldata_utils.h"
+#include "common/relpath.h"
 #include "getopt_long.h"
 #include "pg_getopt.h"
 #include "storage/bufpage.h"
@@ -49,27 +50,69 @@ usage(void)
 	printf(_("Report bugs to <pgsql-bugs@postgresql.org>.\n"));
 }
 
-static const char *const skip[] = {
-	"pg_control",
-	"pg_filenode.map",
-	"pg_internal.init",
-	"PG_VERSION",
-	NULL,
-};
-
+/*
+ * isRelFileName
+ *
+ * Check if the given file name is authorized for checksum verification.
+ */
 static bool
-skipfile(const char *fn)
+isRelFileName(const char *fn)
 {
-	const char *const *f;
-
-	if (strcmp(fn, ".") == 0 ||
-		strcmp(fn, "..") == 0)
+	int			pos;
+
+	/*----------
+	 * Only files including data checksums are authorized for verification.
+	 * This is guessed based on the file name by reverse-engineering
+	 * GetRelationPath() so make sure to update both code paths if any
+	 * updates are done.  The following file name formats are allowed:
+	 * <digits>
+	 * <digits>.<segment>
+	 * <digits>_<forkname>
+	 * <digits>_<forkname>.<segment>
+	 *
+	 * Note that temporary files, beginning with 't', are also skipped.
+	 *
+	 *----------
+	 */
+
+	/* A non-empty string of digits should follow */
+	for (pos = 0; isdigit((unsigned char) fn[pos]); ++pos)
+		;
+	/* leave if no digits */
+	if (pos == 0)
+		return false;
+	/* good to go if only digits */
+	if (fn[pos] == '\0')
 		return true;
 
-	for (f = skip; *f; f++)
-		if (strcmp(*f, fn) == 0)
-			return true;
-	return false;
+	/* Authorized fork files can be scanned */
+	if (fn[pos] == '_')
+	{
+		int			forkchar = forkname_chars(&fn[pos + 1], NULL);
+
+		if (forkchar <= 0)
+			return false;
+
+		pos += forkchar + 1;
+	}
+
+	/* Check for an optional segment number */
+	if (fn[pos] == '.')
+	{
+		int			segchar;
+
+		for (segchar = 1; isdigit((unsigned char) fn[pos + segchar]); ++segchar)
+			;
+
+		if (segchar <= 1)
+			return false;
+		pos += segchar;
+	}
+
+	/* Now this should be the end */
+	if (fn[pos] != '\0')
+		return false;
+	return true;
 }
 
 static void
@@ -146,7 +189,7 @@ scan_directory(const char *basedir, const char *subdir)
 		char		fn[MAXPGPATH];
 		struct stat st;
 
-		if (skipfile(de->d_name))
+		if (!isRelFileName(de->d_name))
 			continue;
 
 		snprintf(fn, sizeof(fn), "%s/%s", path, de->d_name);