Make unsafe greenplum specific function linenumber_atoi() safer

The function was using an anti-pattern where an argument is a static
length char array. Array arguments in C don't really exist but compilers
accept them and without any 'const' or 'static' decorators they don't
always emit a warning.

The proposed patch only fixes the unsafeness of the function
declaration but it does not address the usefulness of it.
Reviewed-by: NDaniel Gustafsson <dgustafsson@pivotal.io>

src/backend/access/external/fileam.c

@@ -1473,7 +1473,7 @@ external_scan_error_callback(void *arg)
 
 		errcontext("External table %s, line %s of %s, column %s",
 				   cstate->cur_relname,
-				   linenumber_atoi(buffer, cstate->cur_lineno),
+				   linenumber_atoi(buffer, sizeof(buffer), cstate->cur_lineno),
 				   scan->fs_uri,
 				   cstate->cur_attname);
 	}
@@ -1489,7 +1489,7 @@ external_scan_error_callback(void *arg)
 
 			errcontext("External table %s, line %s of %s: \"%s\"",
 					   cstate->cur_relname,
-					   linenumber_atoi(buffer, cstate->cur_lineno),
+					   linenumber_atoi(buffer, sizeof(buffer), cstate->cur_lineno),
 					   scan->fs_uri, line_buf);
 			pfree(line_buf);
 		}
@@ -1510,7 +1510,7 @@ external_scan_error_callback(void *arg)
 			if (cstate->cur_lineno > 0)
 				errcontext("External table %s, line %s of file %s",
 						   cstate->cur_relname,
-						   linenumber_atoi(buffer, cstate->cur_lineno),
+						   linenumber_atoi(buffer, sizeof(buffer), cstate->cur_lineno),
 						   scan->fs_uri);
 			else
 				errcontext("External table %s, file %s",
@@ -1600,12 +1600,12 @@ justifyDatabuf(StringInfo buf)
 }
 
 char *
-linenumber_atoi(char buffer[20], int64 linenumber)
+linenumber_atoi(char *buffer, size_t bufsz, int64 linenumber)
 {
 	if (linenumber < 0)
-		return "N/A";
-
-	snprintf(buffer, 20, INT64_FORMAT, linenumber);
+		snprintf(buffer, bufsz, "%s", "N/A");
+	else
+		snprintf(buffer, bufsz, INT64_FORMAT, linenumber);
 
 	return buffer;
 }

src/backend/commands/copy.c

@@ -3274,12 +3274,12 @@ CopyFromErrorCallback(void *arg)
 		if (cstate->cur_attname)
 			errcontext("COPY %s, line %s, column %s",
 					   cstate->cur_relname,
-					   linenumber_atoi(buffer, cstate->cur_lineno),
+					   linenumber_atoi(buffer, sizeof(buffer), cstate->cur_lineno),
 					   cstate->cur_attname);
 		else
 			errcontext("COPY %s, line %s",
 					   cstate->cur_relname,
-					   linenumber_atoi(buffer, cstate->cur_lineno));
+					   linenumber_atoi(buffer, sizeof(buffer), cstate->cur_lineno));
 	}
 	else
 	{
@@ -3291,7 +3291,7 @@ CopyFromErrorCallback(void *arg)
 			attval = limit_printout_length(cstate->cur_attval);
 			errcontext("COPY %s, line %s, column %s: \"%s\"",
 					   cstate->cur_relname,
-					   linenumber_atoi(buffer, cstate->cur_lineno),
+					   linenumber_atoi(buffer, sizeof(buffer), cstate->cur_lineno),
 					   cstate->cur_attname, attval);
 			pfree(attval);
 		}
@@ -3300,7 +3300,7 @@ CopyFromErrorCallback(void *arg)
 			/* error is relevant to a particular column, value is NULL */
 			errcontext("COPY %s, line %s, column %s: null input",
 					   cstate->cur_relname,
-					   linenumber_atoi(buffer, cstate->cur_lineno),
+					   linenumber_atoi(buffer, sizeof(buffer), cstate->cur_lineno),
 					   cstate->cur_attname);
 		}
 		else
@@ -3323,7 +3323,7 @@ CopyFromErrorCallback(void *arg)
 				lineval = limit_printout_length(cstate->line_buf.data);
 				errcontext("COPY %s, line %s: \"%s\"",
 						   cstate->cur_relname,
-						   linenumber_atoi(buffer, cstate->cur_lineno),
+						   linenumber_atoi(buffer, sizeof(buffer), cstate->cur_lineno),
 						   lineval);
 				pfree(lineval);
 			}
@@ -3339,7 +3339,7 @@ CopyFromErrorCallback(void *arg)
 				 */
 				errcontext("COPY %s, line %s",
 						   cstate->cur_relname,
-						   linenumber_atoi(buffer, cstate->cur_lineno));
+						   linenumber_atoi(buffer, sizeof(buffer), cstate->cur_lineno));
 			}
 		}
 	}

src/include/access/fileam.h

@@ -85,7 +85,7 @@ extern ExternalInsertDesc external_insert_init(Relation rel);
 extern Oid	external_insert(ExternalInsertDesc extInsertDesc, HeapTuple instup);
 extern void external_insert_finish(ExternalInsertDesc extInsertDesc);
 extern void external_set_env_vars(extvar_t *extvar, char *uri, bool csv, char *escape, char *quote, bool header, uint32 scancounter);
-extern char *linenumber_atoi(char buffer[20], int64 linenumber);
+extern char *linenumber_atoi(char *buffer, size_t bufsz, int64 linenumber);
 
 /* prototypes for functions in url_execute.c */
 extern int popen_with_stderr(int *rwepipe, const char *exe, bool forwrite);