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authorJonathan Nieder <jrnieder@gmail.com>2014-12-04 03:01:33 (GMT)
committerJunio C Hamano <gitster@pobox.com>2014-12-04 23:27:47 (GMT)
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doc: document error handling functions and conventions
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>
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+Error reporting in git
+======================
+
+`die`, `usage`, `error`, and `warning` report errors of various
+kinds.
+
+- `die` is for fatal application errors. It prints a message to
+ the user and exits with status 128.
+
+- `usage` is for errors in command line usage. After printing its
+ message, it exits with status 129. (See also `usage_with_options`
+ in the link:api-parse-options.html[parse-options API].)
+
+- `error` is for non-fatal library errors. It prints a message
+ to the user and returns -1 for convenience in signaling the error
+ to the caller.
+
+- `warning` is for reporting situations that probably should not
+ occur but which the user (and Git) can continue to work around
+ without running into too many problems. Like `error`, it
+ returns -1 after reporting the situation to the caller.
+
+Customizable error handlers
+---------------------------
+
+The default behavior of `die` and `error` is to write a message to
+stderr and then exit or return as appropriate. This behavior can be
+overridden using `set_die_routine` and `set_error_routine`. For
+example, "git daemon" uses set_die_routine to write the reason `die`
+was called to syslog before exiting.
+
+Library errors
+--------------
+
+Functions return a negative integer on error. Details beyond that
+vary from function to function:
+
+- Some functions return -1 for all errors. Others return a more
+ specific value depending on how the caller might want to react
+ to the error.
+
+- Some functions report the error to stderr with `error`,
+ while others leave that for the caller to do.
+
+- errno is not meaningful on return from most functions (except
+ for thin wrappers for system calls).
+
+Check the function's API documentation to be sure.
+
+Caller-handled errors
+---------------------
+
+An increasing number of functions take a parameter 'struct strbuf *err'.
+On error, such functions append a message about what went wrong to the
+'err' strbuf. The message is meant to be complete enough to be passed
+to `die` or `error` as-is. For example:
+
+ if (ref_transaction_commit(transaction, &err))
+ die("%s", err.buf);
+
+The 'err' parameter will be untouched if no error occured, so multiple
+function calls can be chained:
+
+ t = ref_transaction_begin(&err);
+ if (!t ||
+ ref_transaction_update(t, "HEAD", ..., &err) ||
+ ret_transaction_commit(t, &err))
+ die("%s", err.buf);
+
+The 'err' parameter must be a pointer to a valid strbuf. To silence
+a message, pass a strbuf that is explicitly ignored:
+
+ if (thing_that_can_fail_in_an_ignorable_way(..., &err))
+ /* This failure is okay. */
+ strbuf_reset(&err);