perl: Hide warnings inside perl.h when using gcc compatible compiler

anarazel · anarazel · commit 99f8bc335cbe · 2023-01-02T15:51:05.000-08:00
New versions of perl trigger warnings within perl.h with our compiler flags. At least -Wdeclaration-after-statement, -Wshadow=compatible-local are known to be problematic. To avoid these warnings, conditionally use #pragma GCC system_header before including plperl.h. Alternatively, we could add the include paths for problematic headers with -isystem, but that is a larger hammer and is harder to search for. A more granular alternative would be to use #pragma GCC diagnostic push/ignored/pop, but gcc warns about unknown warnings being ignored, so every to-be-ignored-temporarily compiler warning would require its own pg_config.h symbol and #ifdef. As the warnings are voluminous, it makes sense to backpatch this change. But don't do so yet, we first want gather buildfarm coverage - it's e.g. possible that some compiler claiming to be gcc compatible has issues with the pragma. Author: Andres Freund <andres@anarazel.de> Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us> Discussion: Discussion: https://postgr.es/m/20221228182455.hfdwd22zztvkojy2@awork3.anarazel.de
diff --git a/src/include/c.h b/src/include/c.h
@@ -257,6 +257,34 @@
 #endif
 #endif
 
+/*
+ * Does the compiler support #pragma GCC system_header? We optionally use it
+ * to avoid warnings that we can't fix (e.g. in the perl headers).
+ * See https://gcc.gnu.org/onlinedocs/cpp/System-Headers.html
+ *
+ * Headers for which we do not want to show compiler warnings can,
+ * conditionally, use #pragma GCC system_header to avoid warnings. Obviously
+ * this should only be used for external headers over which we do not have
+ * control.
+ *
+ * Support for the pragma is tested here, instead of during configure, as gcc
+ * also warns about the pragma being used in a .c file. It's surprisingly hard
+ * to get autoconf to use .h as the file-ending. Looks like gcc has
+ * implemented the pragma since the 2000, so this test should suffice.
+ *
+ *
+ * Alternatively, we could add the include paths for problematic headers with
+ * -isystem, but that is a larger hammer and is harder to search for.
+ *
+ * A more granular alternative would be to use #pragma GCC diagnostic
+ * push/ignored/pop, but gcc warns about unknown warnings being ignored, so
+ * every to-be-ignored-temporarily compiler warning would require its own
+ * pg_config.h symbol and #ifdef.
+ */
+#ifdef __GNUC__
+#define HAVE_PRAGMA_GCC_SYSTEM_HEADER	1
+#endif
+
 
 /* ----------------------------------------------------------------
  *				Section 2:	bool, true, false
diff --git a/src/pl/plperl/plperl.h b/src/pl/plperl/plperl.h
@@ -57,6 +57,15 @@
 #define HAS_BOOL 1
 #endif
 
+/*
+ * Newer versions of the perl headers trigger a lot of warnings with our
+ * compiler flags (at least -Wdeclaration-after-statement,
+ * -Wshadow=compatible-local are known to be problematic). The system_header
+ * pragma hides warnings from within the rest of this file, if supported.
+ */
+#ifdef HAVE_PRAGMA_GCC_SYSTEM_HEADER
+#pragma GCC system_header
+#endif
 
 /*
  * Get the basic Perl API.  We use PERL_NO_GET_CONTEXT mode so that our code
