*** 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,
--- 20,33 ----
  #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 call our handler, which jumps out of the parser.
!  */
  #undef fprintf
! #define fprintf(file, fmt, msg) GUC_flex_fatal(msg)
  
  enum {
  	GUC_ID = 1,
***************
*** 37,46 **** enum {
--- 42,54 ----
  };
  
  static unsigned int ConfigFileLineno;
+ static const char *GUC_flex_fatal_errmsg;
+ static sigjmp_buf *GUC_flex_fatal_jmp;
  
  /* flex fails to supply a prototype for yylex, so provide one */
  int GUC_yylex(void);
  
+ static int GUC_flex_fatal(const char *msg);
  static char *GUC_scanstr(const char *s);
  
  %}
***************
*** 437,442 **** ParseConfigFile(const char *config_file, const char *calling_file, bool strict,
--- 445,466 ----
  }
  
  /*
+  * Flex fatal errors bring us here.  Stash the error message and jump back to
+  * ParseConfigFp().  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).  Otherwise, we would need to copy the message.
+  *
+  * We return "int" since this takes the place of calls to fprintf().
+ */
+ static int
+ GUC_flex_fatal(const char *msg)
+ {
+ 	GUC_flex_fatal_errmsg = msg;
+ 	siglongjmp(*GUC_flex_fatal_jmp, 1);
+ 	return 0;	/* keep compiler quiet */
+ }
+ 
+ /*
   * Read and parse a single configuration file.  This function recurses
   * to handle "include" directives.
   *
***************
*** 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()))
  	{
--- 488,525 ----
  			  ConfigVariable **head_p, ConfigVariable **tail_p)
  {
  	bool		OK = true;
! 	unsigned int save_ConfigFileLineno = ConfigFileLineno;
! 	sigjmp_buf *save_GUC_flex_fatal_jmp = GUC_flex_fatal_jmp;
! 	sigjmp_buf	flex_fatal_jmp;
! 	volatile YY_BUFFER_STATE lex_buffer = NULL;
  	int			errorcount;
  	int			token;
  
+ 	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);
+ 
+ 		OK = false;
+ 		goto cleanup;
+ 	}
+ 
  	/*
  	 * 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()))
  	{
***************
*** 526,539 **** ParseConfigFp(FILE *fp, const char *config_file, int depth, int elevel,
  			 * An include_if_exists directive isn't a variable and should be
  			 * processed immediately.
  			 */
- 			unsigned int save_ConfigFileLineno = ConfigFileLineno;
- 
  			if (!ParseConfigFile(opt_value, config_file, false,
  								 depth + 1, elevel,
  								 head_p, tail_p))
  				OK = false;
  			yy_switch_to_buffer(lex_buffer);
- 			ConfigFileLineno = save_ConfigFileLineno;
  			pfree(opt_name);
  			pfree(opt_value);
  		}
--- 569,579 ----
***************
*** 543,556 **** ParseConfigFp(FILE *fp, const char *config_file, int depth, int elevel,
  			 * An include directive isn't a variable and should be processed
  			 * immediately.
  			 */
- 			unsigned int save_ConfigFileLineno = ConfigFileLineno;
- 
  			if (!ParseConfigFile(opt_value, config_file, true,
  								 depth + 1, elevel,
  								 head_p, tail_p))
  				OK = false;
  			yy_switch_to_buffer(lex_buffer);
- 			ConfigFileLineno = save_ConfigFileLineno;
  			pfree(opt_name);
  			pfree(opt_value);
  		}
--- 583,593 ----
***************
*** 620,626 **** ParseConfigFp(FILE *fp, const char *config_file, int depth, int elevel,
--- 657,667 ----
  			break;
  	}
  
+ cleanup:
  	yy_delete_buffer(lex_buffer);
+ 	/* Each recursion level must save and restore these static variables. */
+ 	ConfigFileLineno = save_ConfigFileLineno;
+ 	GUC_flex_fatal_jmp = save_GUC_flex_fatal_jmp;
  	return OK;
  }