From 498287b3c340739bede9a3fa95ea23b6ae650bf3 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Alexander=20F=C3=A6r=C3=B8y?= <ahf@torproject.org>
Date: Mon, 27 Feb 2017 15:37:31 +0100
Subject: [PATCH] Add compatibility function for fgets(3).

This patch adds the `tor_fgets()` function to our compatibility layer.

`tor_fgets()` adds an additional check for whether the error-bit have
been enabled for the given file stream, if that is the case and `errno`
is set to `EAGAIN` we make sure that we always return NULL.

Unfortunately `fgets(3)` behaves differently on different versions of
the C library.

See: https://bugs.torproject.org/21416
See: https://bugs.torproject.org/20988
---
 changes/bug20988    |  4 ++++
 src/common/compat.c | 39 +++++++++++++++++++++++++++++++++++++++
 src/common/compat.h |  2 ++
 3 files changed, 45 insertions(+)
 create mode 100644 changes/bug20988

diff --git a/changes/bug20988 b/changes/bug20988
new file mode 100644
index 0000000000..b1d73f08e7
--- /dev/null
+++ b/changes/bug20988
@@ -0,0 +1,4 @@
+  o Minor bugfixes (portability):
+    - Add Tor compatibility function for fgets(3) due to inconsistency of
+      returned values in different supported C libraries. This fixes unit test
+      failures reported on FreeBSD. Fixes bug 20988.
diff --git a/src/common/compat.c b/src/common/compat.c
index 0dbede6bed..da4283fbaa 100644
--- a/src/common/compat.c
+++ b/src/common/compat.c
@@ -3476,6 +3476,45 @@ tor_getpass(const char *prompt, char *output, size_t buflen)
 #endif
 }
 
+/** Read at most one less than the number of characters specified by
+ * <b>size</b> from the given <b>stream</b> and store it in <b>str</b>.
+ *
+ * Reading stops when a newline character is found or at EOF or error. If any
+ * characters are read and there's no error, a trailing NUL byte is appended to
+ * the end of <b>str</b>.
+ *
+ * Upon successful completion, this function returns a pointer to the string
+ * <b>str</b>. If EOF occurs before any characters are read the function will
+ * return NULL and the content of <b>str</b> is unchanged. Upon error, the
+ * function returns NULL and the caller must check for error using foef(3) and
+ * ferror(3).
+ */
+char *
+tor_fgets(char *str, int size, FILE *stream)
+{
+  char *ret;
+
+  /* Reset errno before our call to fgets(3) to avoid a situation where the
+   * caller is calling us again because we just returned NULL and errno ==
+   * EAGAIN, but when they call us again we will always return NULL because the
+   * error flag on the file handler remains set and errno is set to EAGAIN.
+   */
+  errno = 0;
+
+  ret = fgets(str, size, stream);
+
+  /* FreeBSD, OpenBSD, Linux (glibc), and Linux (musl) seem to disagree about
+   * what to do in the given situation. We check if the stream has been flagged
+   * with an error-bit and return NULL in that situation if errno is also set
+   * to EAGAIN.
+   */
+  if (ferror(stream) && errno == EAGAIN) {
+    return NULL;
+  }
+
+  return ret;
+}
+
 /** Return the amount of free disk space we have permission to use, in
  * bytes. Return -1 if the amount of free space can't be determined. */
 int64_t
diff --git a/src/common/compat.h b/src/common/compat.h
index ee1c9454de..1f51ece61f 100644
--- a/src/common/compat.h
+++ b/src/common/compat.h
@@ -740,6 +740,8 @@ STATIC int tor_ersatz_socketpair(int family, int type, int protocol,
 
 ssize_t tor_getpass(const char *prompt, char *output, size_t buflen);
 
+char *tor_fgets(char *str, int size, FILE *stream);
+
 /* This needs some of the declarations above so we include it here. */
 #include "compat_threads.h"