*** 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,
--- 20,40 ----
  #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.  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,
***************
*** 464,482 **** ParseConfigFp(FILE *fp, const char *config_file, int depth, int elevel,
  			  ConfigVariable **head_p, ConfigVariable **tail_p)
  {
  	bool		OK = true;
! 	YY_BUFFER_STATE lex_buffer;
  	int			errorcount;
  	int			token;
  
  	/*
  	 * Parse
  	 */
- 	lex_buffer = yy_create_buffer(fp, YY_BUF_SIZE);
- 	yy_switch_to_buffer(lex_buffer);
- 
  	ConfigFileLineno = 1;
  	errorcount = 0;
  
  	/* This loop iterates once per logical line */
  	while ((token = yylex()))
  	{
--- 476,511 ----
  			  ConfigVariable **head_p, ConfigVariable **tail_p)
  {
  	bool		OK = true;
! 	volatile YY_BUFFER_STATE lex_buffer = NULL;
  	int			errorcount;
  	int			token;
+ 	sigjmp_buf	flex_fatal_jmp;
+ 
+ 	if (sigsetjmp(flex_fatal_jmp, 1) == 0)
+ 		GUC_flex_fatal_jmp = &flex_fatal_jmp;
+ 	else
+ 	{
+ 		/*
+ 		 * Regain control after a fatal, internal flex error.  It may have
+ 		 * corrupted parser state.  Consequently, abandon the file, but trust
+ 		 * that the state remains sane enough for yy_delete_buffer().
+ 		 */
+ 		elog(elevel, "%s at file \"%s\" line %u",
+ 			 GUC_flex_fatal_errmsg, config_file, ConfigFileLineno);
+ 
+ 		yy_delete_buffer(lex_buffer);
+ 		return false;
+ 	}
  
  	/*
  	 * Parse
  	 */
  	ConfigFileLineno = 1;
  	errorcount = 0;
  
+ 	lex_buffer = yy_create_buffer(fp, YY_BUF_SIZE);
+ 	yy_switch_to_buffer(lex_buffer);
+ 
  	/* This loop iterates once per logical line */
  	while ((token = yylex()))
  	{
***************
*** 532,537 **** ParseConfigFp(FILE *fp, const char *config_file, int depth, int elevel,
--- 561,567 ----
  								 depth + 1, elevel,
  								 head_p, tail_p))
  				OK = false;
+ 			GUC_flex_fatal_jmp = &flex_fatal_jmp;
  			yy_switch_to_buffer(lex_buffer);
  			ConfigFileLineno = save_ConfigFileLineno;
  			pfree(opt_name);
***************
*** 549,554 **** ParseConfigFp(FILE *fp, const char *config_file, int depth, int elevel,
--- 579,585 ----
  								 depth + 1, elevel,
  								 head_p, tail_p))
  				OK = false;
+ 			GUC_flex_fatal_jmp = &flex_fatal_jmp;
  			yy_switch_to_buffer(lex_buffer);
  			ConfigFileLineno = save_ConfigFileLineno;
  			pfree(opt_name);