*** a/src/backend/utils/misc/guc-file.l
--- b/src/backend/utils/misc/guc-file.l
***************
*** 20,28 ****
  #include "utils/guc.h"
  
  
! /* Avoid exit() on fatal scanner errors (a bit ugly -- see yy_fatal_error) */
  #undef fprintf
! #define fprintf(file, fmt, msg)  ereport(ERROR, (errmsg_internal("%s", msg)))
  
  enum {
  	GUC_ID = 1,
--- 20,41 ----
  #include "utils/guc.h"
  
  
! /*
!  * flex emits a yy_fatal_error() function that it calls in response to
!  * critical errors like malloc failure, file I/O errors, and detection of
!  * internal inconsistency.  That function prints a message and calls exit().
!  * Mutate it to instead stash the message and jump out of the parser; see
!  * GUC_safe_yylex() for the other half of the story.  Assume all msg arguments
!  * point to string constants; this holds for flex 2.5.31 (earliest we support)
!  * and flex 2.5.35 (latest as of this writing).
! */
! static const char *GUC_flex_fatal_errmsg;
! static sigjmp_buf GUC_flex_fatal_jmp;
  #undef fprintf
! #define fprintf(file, fmt, msg) \
! 	0; /* eat cast to void */ \
! 	GUC_flex_fatal_errmsg = msg; \
! 	siglongjmp(GUC_flex_fatal_jmp, 1)
  
  enum {
  	GUC_ID = 1,
***************
*** 32,39 **** enum {
  	GUC_EQUALS = 5,
  	GUC_UNQUOTED_STRING = 6,
  	GUC_QUALIFIED_ID = 7,
! 	GUC_EOL = 99,
! 	GUC_ERROR = 100
  };
  
  static unsigned int ConfigFileLineno;
--- 45,53 ----
  	GUC_EQUALS = 5,
  	GUC_UNQUOTED_STRING = 6,
  	GUC_QUALIFIED_ID = 7,
! 	GUC_EOL = 98,
! 	GUC_ERROR = 99,
! 	GUC_INTERNAL_ERROR = 100
  };
  
  static unsigned int ConfigFileLineno;
***************
*** 437,442 **** ParseConfigFile(const char *config_file, const char *calling_file, bool strict,
--- 451,469 ----
  }
  
  /*
+  * Wrap the generated lexer so we can regain control after a fatal, internal
+  * flex error.  On return of GUC_INTERNAL_ERROR, GUC_flex_fatal_errmsg will
+  * contain a pointer into static storage containing the error message.
+  */
+ static int
+ GUC_safe_yylex(void)
+ {
+ 	if (sigsetjmp(GUC_flex_fatal_jmp, 1) != 0)
+ 		return GUC_INTERNAL_ERROR;
+ 	return GUC_yylex();
+ }
+ 
+ /*
   * Read and parse a single configuration file.  This function recurses
   * to handle "include" directives.
   *
***************
*** 478,484 **** ParseConfigFp(FILE *fp, const char *config_file, int depth, int elevel,
  	errorcount = 0;
  
  	/* This loop iterates once per logical line */
! 	while ((token = yylex()))
  	{
  		char	   *opt_name = NULL;
  		char	   *opt_value = NULL;
--- 505,511 ----
  	errorcount = 0;
  
  	/* This loop iterates once per logical line */
! 	while ((token = GUC_safe_yylex()))
  	{
  		char	   *opt_name = NULL;
  		char	   *opt_value = NULL;
***************
*** 493,501 **** ParseConfigFp(FILE *fp, const char *config_file, int depth, int elevel,
  		opt_name = pstrdup(yytext);
  
  		/* next we have an optional equal sign; discard if present */
! 		token = yylex();
  		if (token == GUC_EQUALS)
! 			token = yylex();
  
  		/* now we must have the option value */
  		if (token != GUC_ID &&
--- 520,528 ----
  		opt_name = pstrdup(yytext);
  
  		/* next we have an optional equal sign; discard if present */
! 		token = GUC_safe_yylex();
  		if (token == GUC_EQUALS)
! 			token = GUC_safe_yylex();
  
  		/* now we must have the option value */
  		if (token != GUC_ID &&
***************
*** 510,516 **** ParseConfigFp(FILE *fp, const char *config_file, int depth, int elevel,
  			opt_value = pstrdup(yytext);
  
  		/* now we'd like an end of line, or possibly EOF */
! 		token = yylex();
  		if (token != GUC_EOL)
  		{
  			if (token != 0)
--- 537,543 ----
  			opt_value = pstrdup(yytext);
  
  		/* now we'd like an end of line, or possibly EOF */
! 		token = GUC_safe_yylex();
  		if (token != GUC_EOL)
  		{
  			if (token != 0)
***************
*** 583,589 **** ParseConfigFp(FILE *fp, const char *config_file, int depth, int elevel,
  			pfree(opt_value);
  
  		/* report the error */
! 		if (token == GUC_EOL || token == 0)
  			ereport(elevel,
  					(errcode(ERRCODE_SYNTAX_ERROR),
  					 errmsg("syntax error in file \"%s\" line %u, near end of line",
--- 610,619 ----
  			pfree(opt_value);
  
  		/* report the error */
! 		if (token == GUC_INTERNAL_ERROR)
! 			elog(elevel, "%s at file \"%s\" line %u",
! 				 GUC_flex_fatal_errmsg, config_file, ConfigFileLineno);
! 		else if (token == GUC_EOL || token == 0)
  			ereport(elevel,
  					(errcode(ERRCODE_SYNTAX_ERROR),
  					 errmsg("syntax error in file \"%s\" line %u, near end of line",
***************
*** 612,622 **** ParseConfigFp(FILE *fp, const char *config_file, int depth, int elevel,
  			break;
  		}
  
! 		/* resync to next end-of-line or EOF */
! 		while (token != GUC_EOL && token != 0)
! 			token = yylex();
! 		/* break out of loop on EOF */
! 		if (token == 0)
  			break;
  	}
  
--- 642,655 ----
  			break;
  		}
  
! 		/*
! 		 * Resync to next end-of-line or EOF.  Abandon the file after a
! 		 * flex-internal error, which may corrupt parser state.  Hopefully,
! 		 * it's not so corrupt that yy_delete_buffer() will explode...
! 		 */
! 		while (token != GUC_EOL && token != 0 && token != GUC_INTERNAL_ERROR)
! 			token = GUC_safe_yylex();
! 		if (token != GUC_EOL)
  			break;
  	}