[RFC PATCH v2 1/2] util: Add thread-safe qemu_strerror() function

[Yohei Kojima]

Add qemu_strerror() which follows the POSIX specification for
strerror(). While strerror() is not guaranteed to be thread-safe, this
function is thread-safe.

This function is added to solve the following issue:
https://gitlab.com/qemu-project/qemu/-/issues/416

Signed-off-by: Yohei Kojima <y-koj@outlook.jp>
---
 include/qemu/cutils.h | 20 +++++++++++++++++++
 util/cutils.c         | 45 +++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 65 insertions(+)

diff --git a/include/qemu/cutils.h b/include/qemu/cutils.h
index 92c436d8c7..0bcae0049a 100644
--- a/include/qemu/cutils.h
+++ b/include/qemu/cutils.h
@@ -117,6 +117,26 @@ int stristart(const char *str, const char *val, const char 
**ptr);
  * Returns: length of @s in bytes, or @max_len, whichever is smaller.
  */
 int qemu_strnlen(const char *s, int max_len);
+/**
+ * qemu_strerror:
+ * @errnum: error number
+ *
+ * Return the pointer to a string that describes errnum, like
+ * strerror(). This function is thread-safe because the buffer for
+ * returned string is allocated per thread.
+ *
+ * This function is thread-safe, but not reentrant. In other words,
+ * if a thread is interrupted by a signal in this function, and the
+ * thread calls this function again in the signal handling, then
+ * the result might be corrupted.
+ *
+ * This function has the same behaviour as the POSIX strerror()
+ * function.
+ *
+ * Returns: the pointer to an error description, or an
+ * "Unknown error nnn" message if errnum is invalid.
+ */
+char *qemu_strerror(int errnum);
 /**
  * qemu_strsep:
  * @input: pointer to string to parse
diff --git a/util/cutils.c b/util/cutils.c
index 5887e74414..3d14f50c75 100644
--- a/util/cutils.c
+++ b/util/cutils.c
@@ -131,6 +131,51 @@ int qemu_strnlen(const char *s, int max_len)
     return i;
 }
 
+/**
+ * It assumes the length of error descriptions are at most 1024.
+ * The concern of write buffer overflow is cared by strerror_r().
+ */
+static __thread char qemu_strerror_buf[1024];
+
+#if (_POSIX_C_SOURCE >= 200112L) && ! _GNU_SOURCE
+/**
+ * In POSIX.1-2001 and after, the return type of strerror_r is int, but
+ * glibc overrides the definition of strerror_r to the old strerror_r
+ * if _GNU_SOURCE is defined. This condition handles it.
+ */
+
+char *qemu_strerror(int errnum)
+{
+    int is_error = strerror_r(errnum, qemu_strerror_buf, 1024);
+
+    if (is_error == 0) return qemu_strerror_buf;
+
+    /**
+     * handle the error occured in strerror_r
+     *
+     * If is_error is greater than 0, errno might not be updated by
+     * strerror_r. Otherwise, errno is updated.
+     */
+    if (is_error > 0) errno = is_error;
+
+    strncpy(qemu_strerror_buf, "Error %d occured\n", errno);
+    return qemu_strerror_buf;
+}
+#else
+/**
+ * In glibc, the return type of strerror_r is char* if _GNU_SOURCE
+ * is defined. In this case, strerror_r returns qemu_strerror_buf iff
+ * some error occured in strerror_r, and otherwise it returns a pointer
+ * to the pre-defined description for errnum.
+ *
+ * This is the same behaviour until POSIX.1-2001.
+ */
+char *qemu_strerror(int errnum)
+{
+    return strerror_r(errnum, qemu_strerror_buf, 1024);
+}
+#endif
+
 char *qemu_strsep(char **input, const char *delim)
 {
     char *result = *input;
-- 
2.39.2