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

@@ -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.

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

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"