Add backtrace support for error reporting

Add some support for automatically showing backtraces in certain error
situations in the server.  Backtraces are shown on assertion failure;
also, a new setting backtrace_functions can be set to a list of C
function names, and all ereport()s and elog()s from the mentioned
functions will have backtraces generated.  Finally, the function
errbacktrace() can be manually added to an ereport() call to generate a
backtrace for that call.

Authors: Peter Eisentraut, Álvaro Herrera
Discussion: https://postgr.es/m//5f48cb47-bf1e-05b6-7aae-3bf2cd01586d@2ndquadrant.com
Discussion: https://postgr.es/m/CAMsr+YGL+yfWE=JvbUbnpWtrRZNey7hJ07+zT4bYJdVp4Szdrg@mail.gmail.com

src/backend/utils/misc/guc.c

@@ -201,6 +201,8 @@ static bool check_cluster_name(char **newval, void **extra, GucSource source);
 static const char *show_unix_socket_permissions(void);
 static const char *show_log_file_mode(void);
 static const char *show_data_directory_mode(void);
+static bool check_backtrace_functions(char **newval, void **extra, GucSource source);
+static void assign_backtrace_functions(const char *newval, void *extra);
 static bool check_recovery_target_timeline(char **newval, void **extra, GucSource source);
 static void assign_recovery_target_timeline(const char *newval, void *extra);
 static bool check_recovery_target(char **newval, void **extra, GucSource source);
@@ -515,6 +517,8 @@ int			log_temp_files = -1;
 double		log_statement_sample_rate = 1.0;
 double		log_xact_sample_rate = 0;
 int			trace_recovery_messages = LOG;
+char	   *backtrace_functions;
+char	   *backtrace_symbol_list;
 
 int			temp_file_limit = -1;
 
@@ -4224,6 +4228,17 @@ static struct config_string ConfigureNamesString[] =
 		NULL, NULL, NULL
 	},
 
+	{
+		{"backtrace_functions", PGC_SUSET, DEVELOPER_OPTIONS,
+			gettext_noop("Log backtrace for errors in these functions."),
+			NULL,
+			GUC_NOT_IN_SAMPLE
+		},
+		&backtrace_functions,
+		"",
+		check_backtrace_functions, assign_backtrace_functions, NULL
+	},
+
 	/* End-of-list marker */
 	{
 		{NULL, 0, 0, NULL, NULL}, NULL, NULL, NULL, NULL, NULL
@@ -11487,6 +11502,76 @@ show_data_directory_mode(void)
 	return buf;
 }
 
+/*
+ * We split the input string, where commas separate function names
+ * and certain whitespace chars are ignored, into a \0-separated (and
+ * \0\0-terminated) list of function names.  This formulation allows
+ * easy scanning when an error is thrown while avoiding the use of
+ * non-reentrant strtok(), as well as keeping the output data in a
+ * single palloc() chunk.
+ */
+static bool
+check_backtrace_functions(char **newval, void **extra, GucSource source)
+{
+	int			newvallen = strlen(*newval);
+	char	   *someval;
+	int			validlen;
+	int			i;
+	int			j;
+
+	/*
+	 * Allow characters that can be C identifiers and commas as separators, as
+	 * well as some whitespace for readability.
+	 */
+	validlen = strspn(*newval,
+					  "0123456789_"
+					  "abcdefghijklmnopqrstuvwxyz"
+					  "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
+					  ", \n\t");
+	if (validlen != newvallen)
+	{
+		GUC_check_errdetail("invalid character");
+		return false;
+	}
+
+	if (*newval[0] == '\0')
+	{
+		*extra = NULL;
+		return true;
+	}
+
+	/*
+	 * Allocate space for the output and create the copy.  We could discount
+	 * whitespace chars to save some memory, but it doesn't seem worth the
+	 * trouble.
+	 */
+	someval = guc_malloc(ERROR, newvallen + 1 + 1);
+	for (i = 0, j = 0; i < newvallen; i++)
+	{
+		if ((*newval)[i] == ',')
+			someval[j++] = '\0';	/* next item */
+		else if ((*newval)[i] == ' ' ||
+				 (*newval)[i] == '\n' ||
+				 (*newval)[i] == '\t')
+			;	/* ignore these */
+		else
+			someval[j++] = (*newval)[i];	/* copy anything else */
+	}
+
+	/* two \0s end the setting */
+	someval[j] = '\0';
+	someval[j + 1] = '\0';
+
+	*extra = someval;
+	return true;
+}
+
+static void
+assign_backtrace_functions(const char *newval, void *extra)
+{
+	backtrace_symbol_list = (char *) extra;
+}
+
 static bool
 check_recovery_target_timeline(char **newval, void **extra, GucSource source)
 {