Add GUC backtrace_on_internal_error

When enabled (default off), this logs a backtrace anytime elog() or an
equivalent ereport() for internal errors is called.

This is not well covered by the existing backtrace_functions, because
there are many equally-worded low-level errors in many functions.  And
if you find out where the error is, then you need to manually rewrite
the elog() to ereport() to attach the errbacktrace(), which is
annoying.  Having a backtrace automatically on every elog() call could
be very helpful during development for various kinds of common errors
from palloc, syscache, node support, etc.

Discussion: https://www.postgresql.org/message-id/flat/ba76c6bc-f03f-4285-bf16-47759cfcab9e@eisentraut.org

doc/src/sgml/config.sgml

@@ -11086,6 +11086,33 @@ dynamic_library_path = 'C:\tools\postgresql;H:\my_project\lib;$libdir'
       </listitem>
      </varlistentry>
 
+     <varlistentry id="guc-backtrace-on-internal-error" xreflabel="backtrace_on_internal_error">
+      <term><varname>backtrace_on_internal_error</varname> (<type>boolean</type>)
+      <indexterm>
+        <primary><varname>backtrace_on_internal_error</varname> configuration parameter</primary>
+      </indexterm>
+      </term>
+      <listitem>
+       <para>
+        If this parameter is on and an error with error code XX000 (internal
+        error; see also <xref linkend="errcodes-appendix"/>) is raised, then a
+        backtrace is written to the server log together with the error
+        message.  This can be used to debug such internal errors (which should
+        normally not happen in production).  The default is off.
+       </para>
+
+       <para>
+        Backtrace support is not available on all platforms, and the quality
+        of the backtraces depends on compilation options.
+       </para>
+
+       <para>
+        Only superusers and users with the appropriate <literal>SET</literal>
+        privilege can change this setting.
+       </para>
+      </listitem>
+     </varlistentry>
+
      <varlistentry id="guc-debug-discard-caches" xreflabel="debug_discard_caches">
       <term><varname>debug_discard_caches</varname> (<type>integer</type>)
       <indexterm>

src/backend/utils/error/elog.c

@@ -498,9 +498,11 @@ errfinish(const char *filename, int lineno, const char *funcname)
 
 	/* Collect backtrace, if enabled and we didn't already */
 	if (!edata->backtrace &&
-		edata->funcname &&
-		backtrace_functions &&
-		matches_backtrace_functions(edata->funcname))
+		((edata->funcname &&
+		  backtrace_functions &&
+		  matches_backtrace_functions(edata->funcname)) ||
+		 (edata->sqlerrcode == ERRCODE_INTERNAL_ERROR &&
+		  backtrace_on_internal_error)))
 		set_backtrace(edata, 2);
 
 	/*

src/backend/utils/misc/guc_tables.c

@@ -527,6 +527,7 @@ int			log_temp_files = -1;
 double		log_statement_sample_rate = 1.0;
 double		log_xact_sample_rate = 0;
 char	   *backtrace_functions;
+bool		backtrace_on_internal_error = false;
 
 int			temp_file_limit = -1;
 
@@ -812,6 +813,16 @@ StaticAssertDecl(lengthof(config_type_names) == (PGC_ENUM + 1),
 
 struct config_bool ConfigureNamesBool[] =
 {
+	{
+		{"backtrace_on_internal_error", PGC_SUSET, DEVELOPER_OPTIONS,
+			gettext_noop("Log backtrace for any error with error code XX000 (internal error)."),
+			NULL,
+			GUC_NOT_IN_SAMPLE
+		},
+		&backtrace_on_internal_error,
+		false,
+		NULL, NULL, NULL
+	},
 	{
 		{"enable_seqscan", PGC_USERSET, QUERY_TUNING_METHOD,
 			gettext_noop("Enables the planner's use of sequential-scan plans."),

src/include/utils/guc.h

@@ -266,6 +266,7 @@ extern PGDLLIMPORT int log_temp_files;
 extern PGDLLIMPORT double log_statement_sample_rate;
 extern PGDLLIMPORT double log_xact_sample_rate;
 extern PGDLLIMPORT char *backtrace_functions;
+extern PGDLLIMPORT bool backtrace_on_internal_error;
 
 extern PGDLLIMPORT int temp_file_limit;