error: Revamp interface documentation

Backports commit edf6f3b3358597d37da0cf636ce3ed8a546d0f26 from qemu

qemu/include/qapi/error.h

@@ -2,13 +2,75 @@
  * QEMU Error Objects
  *
  * Copyright IBM, Corp. 2011
+ * Copyright (C) 2011-2015 Red Hat, Inc.
  *
  * Authors:
  *  Anthony Liguori   <aliguori@us.ibm.com>
+ *  Markus Armbruster <armbru@redhat.com>
  *
  * This work is licensed under the terms of the GNU LGPL, version 2.  See
  * the COPYING.LIB file in the top-level directory.
  */
+
+/*
+ * Error reporting system loosely patterned after Glib's GError.
+ *
+ * Create an error:
+ *     error_setg(&err, "situation normal, all fouled up");
+ *
+ * Report an error to stderr:
+ *     error_report_err(err);
+ * This frees the error object.
+ *
+ * Report an error somewhere else:
+ *     const char *msg = error_get_pretty(err);
+ *     do with msg what needs to be done...
+ *     error_free(err);
+ *
+ * Handle an error without reporting it (just for completeness):
+ *     error_free(err);
+ *
+ * Pass an existing error to the caller:
+ *     error_propagate(errp, err);
+ * where Error **errp is a parameter, by convention the last one.
+ *
+ * Create a new error and pass it to the caller:
+ *     error_setg(errp, "situation normal, all fouled up");
+ *
+ * Call a function and receive an error from it:
+ *     Error *err = NULL;
+ *     foo(arg, &err);
+ *     if (err) {
+ *         handle the error...
+ *     }
+ *
+ * Call a function ignoring errors:
+ *     foo(arg, NULL);
+ *
+ * Call a function aborting on errors:
+ *     foo(arg, &error_abort);
+ *
+ * Receive an error and pass it on to the caller:
+ *     Error *err = NULL;
+ *     foo(arg, &err);
+ *     if (err) {
+ *         handle the error...
+ *         error_propagate(errp, err);
+ *     }
+ * where Error **errp is a parameter, by convention the last one.
+ *
+ * Do *not* "optimize" this to
+ *     foo(arg, errp);
+ *     if (*errp) { // WRONG!
+ *         handle the error...
+ *     }
+ * because errp may be NULL!
+ *
+ * But when all you do with the error is pass it on, please use
+ *     foo(arg, errp);
+ * for readability.
+ */
+
 #ifndef ERROR_H
 #define ERROR_H
 
@@ -17,18 +79,21 @@
 #include "unicorn/platform.h"
 
 /**
- * A class representing internal errors within QEMU.  An error has a ErrorClass
- * code and a human message.
+ * Opaque error object.
  */
 typedef struct Error Error;
 
 /**
- * Set an indirect pointer to an error given a ErrorClass value and a
- * printf-style human message.  This function is not meant to be used outside
- * of QEMU.
+ * Get @err's human-readable error message.
  */
-void error_set(Error **errp, ErrorClass err_class, const char *fmt, ...)
-    GCC_FMT_ATTR(3, 4);
+const char *error_get_pretty(Error *err);
+
+/*
+ * Get @err's error class.
+ * Note: use of error classes other than ERROR_CLASS_GENERIC_ERROR is
+ * strongly discouraged.
+ */
+ErrorClass error_get_class(const Error *err);
 
 /**
  * Set an indirect pointer to an error given a ErrorClass value and a
@@ -38,50 +103,68 @@ void error_set(Error **errp, ErrorClass err_class, const char *fmt, ...)
 void error_set_errno(Error **errp, int os_error, ErrorClass err_class,
                      const char *fmt, ...) GCC_FMT_ATTR(4, 5);
 
-/**
- * Same as error_set(), but sets a generic error
+/*
+ * Create a new error object and assign it to *@errp.
+ * If @errp is NULL, the error is ignored.  Don't bother creating one
+ * then.
+ * If @errp is &error_abort, print a suitable message and abort().
+ * If @errp is anything else, *@errp must be NULL.
+ * The new error's class is ERROR_CLASS_GENERIC_ERROR, and its
+ * human-readable error message is made from printf-style @fmt, ...
  */
 void error_setg(Error **errp, const char *fmt, ...)
     GCC_FMT_ATTR(2, 3);
+
+/*
+ * Just like error_setg(), with @os_error info added to the message.
+ * If @os_error is non-zero, ": " + strerror(os_error) is appended to
+ * the human-readable error message.
+ */
 void error_setg_errno(Error **errp, int os_error, const char *fmt, ...)
     GCC_FMT_ATTR(3, 4);
 
-/**
- * Helper for open() errors
- */
-void error_setg_file_open(Error **errp, int os_errno, const char *filename);
-
 /*
- * Get the error class of an error object.
- */
-ErrorClass error_get_class(const Error *err);
-
-/**
- * Returns an exact copy of the error passed as an argument.
- */
-Error *error_copy(const Error *err);
-
-/**
- * Get a human readable representation of an error object.
- */
-const char *error_get_pretty(Error *err);
-
-/**
- * Propagate an error to an indirect pointer to an error.  This function will
- * always transfer ownership of the error reference and handles the case where
- * dst_err is NULL correctly.  Errors after the first are discarded.
+ * Propagate error object (if any) from @local_err to @dst_errp.
+ * If @local_err is NULL, do nothing (because there's nothing to
+ * propagate).
+ * Else, if @dst_errp is NULL, errors are being ignored.  Free the
+ * error object.
+ * Else, if @dst_errp is &error_abort, print a suitable message and
+ * abort().
+ * Else, if @dst_errp already contains an error, ignore this one: free
+ * the error object.
+ * Else, move the error object from @local_err to *@dst_errp.
+ * On return, @local_err is invalid.
  */
 void error_propagate(Error **dst_errp, Error *local_err);
 
 /**
- * Free an error object.
+ * Convenience function to report open() failure.
+ */
+void error_setg_file_open(Error **errp, int os_errno, const char *filename);
+
+/**
+ * Return an exact copy of @err.
+ */
+Error *error_copy(const Error *err);
+
+/**
+ * Free @err.
+ * @err may be NULL.
  */
 void error_free(Error *err);
 
-/**
- * If passed to error_set and friends, abort().
+/*
+ * Just like error_setg(), except you get to specify the error class.
+ * Note: use of error classes other than ERROR_CLASS_GENERIC_ERROR is
+ * strongly discouraged.
  */
+void error_set(Error **errp, ErrorClass err_class, const char *fmt, ...)
+    GCC_FMT_ATTR(3, 4);
 
+/*
+ * Pass to error_setg() & friends to abort() on error.
+ */
 extern Error *error_abort;
 
 #endif