Rethink API for pg_get_line.c, one more time.

Further experience says that the appending behavior offered by
pg_get_line_append is useful to only a very small minority of callers.
For most, the requirement to reset the buffer after each line is just
an error-prone nuisance.  Hence, invent another alternative call
pg_get_line_buf, which takes care of that detail.

Noted while reviewing a patch from Daniel Gustafsson.

Discussion: https://postgr.es/m/48A4FA71-524E-41B9-953A-FD04EF36E2E7@yesql.se

src/bin/initdb/initdb.c

@@ -486,7 +486,7 @@ readfile(const char *path)
 	result = (char **) pg_malloc(maxlines * sizeof(char *));
 
 	n = 0;
-	while (pg_get_line_append(infile, &line))
+	while (pg_get_line_buf(infile, &line))
 	{
 		/* make sure there will be room for a trailing NULL pointer */
 		if (n >= maxlines - 1)
@@ -496,8 +496,6 @@ readfile(const char *path)
 		}
 
 		result[n++] = pg_strdup(line.data);
-
-		resetStringInfo(&line);
 	}
 	result[n] = NULL;
 

src/common/pg_get_line.c

@@ -45,7 +45,8 @@
  * Also note that the palloc'd buffer is usually a lot longer than
  * strictly necessary, so it may be inadvisable to use this function
  * to collect lots of long-lived data.  A less memory-hungry option
- * is to use pg_get_line_append() in a loop, then pstrdup() each line.
+ * is to use pg_get_line_buf() or pg_get_line_append() in a loop,
+ * then pstrdup() each line.
  */
 char *
 pg_get_line(FILE *stream)
@@ -67,11 +68,37 @@ pg_get_line(FILE *stream)
 	return buf.data;
 }
 
+/*
+ * pg_get_line_buf()
+ *
+ * This has similar behavior to pg_get_line(), and thence to fgets(),
+ * except that the collected data is returned in a caller-supplied
+ * StringInfo buffer.  This is a convenient API for code that just
+ * wants to read and process one line at a time, without any artificial
+ * limit on line length.
+ *
+ * Returns true if a line was successfully collected (including the
+ * case of a non-newline-terminated line at EOF).  Returns false if
+ * there was an I/O error or no data was available before EOF.
+ * (Check ferror(stream) to distinguish these cases.)
+ *
+ * In the false-result case, buf is reset to empty.
+ */
+bool
+pg_get_line_buf(FILE *stream, StringInfo buf)
+{
+	/* We just need to drop any data from the previous call */
+	resetStringInfo(buf);
+	return pg_get_line_append(stream, buf);
+}
+
 /*
  * pg_get_line_append()
  *
  * This has similar behavior to pg_get_line(), and thence to fgets(),
  * except that the collected data is appended to whatever is in *buf.
+ * This is useful in preference to pg_get_line_buf() if the caller wants
+ * to merge some lines together, e.g. to implement backslash continuation.
  *
  * Returns true if a line was successfully collected (including the
  * case of a non-newline-terminated line at EOF).  Returns false if

src/include/common/string.h

@@ -21,6 +21,7 @@ extern int	pg_strip_crlf(char *str);
 
 /* functions in src/common/pg_get_line.c */
 extern char *pg_get_line(FILE *stream);
+extern bool pg_get_line_buf(FILE *stream, struct StringInfoData *buf);
 extern bool pg_get_line_append(FILE *stream, struct StringInfoData *buf);
 
 /* functions in src/common/sprompt.c */

src/interfaces/ecpg/test/pg_regress_ecpg.c

@@ -49,7 +49,7 @@ ecpg_filter(const char *sourcefile, const char *outfile)
 
 	initStringInfo(&linebuf);
 
-	while (pg_get_line_append(s, &linebuf))
+	while (pg_get_line_buf(s, &linebuf))
 	{
 		/* check for "#line " in the beginning */
 		if (strstr(linebuf.data, "#line ") == linebuf.data)
@@ -69,7 +69,6 @@ ecpg_filter(const char *sourcefile, const char *outfile)
 			}
 		}
 		fputs(linebuf.data, t);
-		resetStringInfo(&linebuf);
 	}
 
 	pfree(linebuf.data);

src/test/regress/pg_regress.c

@@ -566,7 +566,7 @@ convert_sourcefiles_in(const char *source_subdir, const char *dest_dir, const ch
 
 		initStringInfo(&line);
 
-		while (pg_get_line_append(infile, &line))
+		while (pg_get_line_buf(infile, &line))
 		{
 			replace_string(&line, "@abs_srcdir@", inputdir);
 			replace_string(&line, "@abs_builddir@", outputdir);
@@ -574,7 +574,6 @@ convert_sourcefiles_in(const char *source_subdir, const char *dest_dir, const ch
 			replace_string(&line, "@libdir@", dlpath);
 			replace_string(&line, "@DLSUFFIX@", DLSUFFIX);
 			fputs(line.data, outfile);
-			resetStringInfo(&line);
 		}
 
 		pfree(line.data);