fix(compat): drop fgetln compat shim, migrate prompt-history.c to getline

Following up on the comment on #5365 - this drops fgetln from
prompt-history.c (its only remaining caller) in favor of getline, and
removes the fgetln compat shim entirely since nothing references it
anymore: the compat.h declaration, compat/fgetln.c itself, and its
entry in configure.ac's AC_REPLACE_FUNCS list.

fgetln doesn't null-terminate its returned buffer when the final line
in the file lacks a trailing newline, so the old loop had a second
branch to heap-copy and manually terminate that case. getline always
null-terminates regardless, so that branch goes away and the loop
becomes uniform.

Verified with a real build (ubuntu-24.04 container, same
--enable-utf8proc --enable-asan configure CI uses): crafted a history
file with a normal entry, a blank line, and a final entry with no
trailing newline at EOF (the exact case the removed branch existed
for), loaded it through a real ASan-built tmux via show-prompt-history,
all three entries came back correctly. Full regress suite is otherwise
clean against this change.

compat.h

@@ -410,11 +410,6 @@ int		 asprintf(char **, const char *, ...);
 int		 vasprintf(char **, const char *, va_list);
 #endif
 
-#ifndef HAVE_FGETLN
-/* fgetln.c */
-char		*fgetln(FILE *, size_t *);
-#endif
-
 #ifndef HAVE_GETLINE
 /* getline.c */
 ssize_t		 getline(char **, size_t *, FILE *);

compat/fgetln.c

@@ -1,61 +0,0 @@
-/*
- * Copyright (c) 2015 Joerg Jung <jung@openbsd.org>
- *
- * Permission to use, copy, modify, and distribute this software for any
- * purpose with or without fee is hereby granted, provided that the above
- * copyright notice and this permission notice appear in all copies.
- *
- * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
- * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
- * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
- * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
- * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
- * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
- * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
- */
-
-/*
- * portable fgetln() version, NOT reentrant
- */
-
-#include <stdio.h>
-#include <stdlib.h>
-#include <errno.h>
-
-#include "compat.h"
-
-char *
-fgetln(FILE *fp, size_t *len)
-{
-	static char *buf = NULL;
-	static size_t bufsz = 0;
-	size_t r = 0;
-	char *p;
-	int c, e;
-
-	if (!fp || !len) {
-		errno = EINVAL;
-		return NULL;
-	}
-	if (!buf) {
-		if (!(buf = calloc(1, BUFSIZ)))
-			return NULL;
-		bufsz = BUFSIZ;
-	}
-	while ((c = getc(fp)) != EOF) {
-		buf[r++] = c;
-		if (r == bufsz) {
-			if (!(p = reallocarray(buf, 2, bufsz))) {
-				e = errno;
-				free(buf);
-				errno = e;
-				buf = NULL, bufsz = 0;
-				return NULL;
-			}
-			buf = p, bufsz = 2 * bufsz;
-		}
-		if (c == '\n')
-			break;
-	}
-	return (*len = r) ? buf : NULL;
-}

configure.ac

@@ -177,7 +177,6 @@ AC_REPLACE_FUNCS([ \
 	clock_gettime \
 	closefrom \
 	explicit_bzero \
-	fgetln \
 	freezero \
 	getdtablecount \
 	getdtablesize \

prompt-history.c

@@ -79,8 +79,9 @@ void
 prompt_load_history(void)
 {
 	FILE	*f;
-	char	*history_file, *line, *tmp;
+	char	*history_file, *line = NULL;
-	size_t	 length;
+	size_t	 length = 0;
+	ssize_t	 got;
 
 	if ((history_file = prompt_find_history_file()) == NULL)
 		return;
@@ -94,23 +95,13 @@ prompt_load_history(void)
 	}
 	free(history_file);
 
-	for (;;) {
+	while ((got = getline(&line, &length, f)) != -1) {
-		if ((line = fgetln(f, &length)) == NULL)
+		if (got > 0 && line[got - 1] == '\n')
-			break;
+			line[got - 1] = '\0';
-
+		if (got > 0)
-		if (length > 0) {
+			prompt_add_typed_history(line);
-			if (line[length - 1] == '\n') {
-				line[length - 1] = '\0';
-				prompt_add_typed_history(line);
-			} else {
-				tmp = xmalloc(length + 1);
-				memcpy(tmp, line, length);
-				tmp[length] = '\0';
-				prompt_add_typed_history(tmp);
-				free(tmp);
-			}
-		}
 	}
+	free(line);
 	fclose(f);
 }