merge(3p/git): Merge git upstream at v2.26.2

third_party/git/run-command.h

@@ -5,8 +5,60 @@
 
 #include "argv-array.h"
 
+/**
+ * The run-command API offers a versatile tool to run sub-processes with
+ * redirected input and output as well as with a modified environment
+ * and an alternate current directory.
+ *
+ * A similar API offers the capability to run a function asynchronously,
+ * which is primarily used to capture the output that the function
+ * produces in the caller in order to process it.
+ */
+
+
+/**
+ * This describes the arguments, redirections, and environment of a
+ * command to run in a sub-process.
+ *
+ * The caller:
+ *
+ * 1. allocates and clears (using child_process_init() or
+ *    CHILD_PROCESS_INIT) a struct child_process variable;
+ * 2. initializes the members;
+ * 3. calls start_command();
+ * 4. processes the data;
+ * 5. closes file descriptors (if necessary; see below);
+ * 6. calls finish_command().
+ *
+ * Special forms of redirection are available by setting these members
+ * to 1:
+ *
+ *  .no_stdin, .no_stdout, .no_stderr: The respective channel is
+ *		redirected to /dev/null.
+ *
+ *	.stdout_to_stderr: stdout of the child is redirected to its
+ *		stderr. This happens after stderr is itself redirected.
+ *		So stdout will follow stderr to wherever it is
+ *		redirected.
+ */
 struct child_process {
+
+	/**
+	 * The .argv member is set up as an array of string pointers (NULL
+	 * terminated), of which .argv[0] is the program name to run (usually
+	 * without a path). If the command to run is a git command, set argv[0] to
+	 * the command name without the 'git-' prefix and set .git_cmd = 1.
+	 *
+	 * Note that the ownership of the memory pointed to by .argv stays with the
+	 * caller, but it should survive until `finish_command` completes. If the
+	 * .argv member is NULL, `start_command` will point it at the .args
+	 * `argv_array` (so you may use one or the other, but you must use exactly
+	 * one). The memory in .args will be cleaned up automatically during
+	 * `finish_command` (or during `start_command` when it is unsuccessful).
+	 *
+	 */
 	const char **argv;
+
 	struct argv_array args;
 	struct argv_array env_array;
 	pid_t pid;
@@ -18,8 +70,8 @@ struct child_process {
 
 	/*
 	 * Using .in, .out, .err:
-	 * - Specify 0 for no redirections (child inherits stdin, stdout,
-	 *   stderr from parent).
+	 * - Specify 0 for no redirections. No new file descriptor is allocated.
+	 * (child inherits stdin, stdout, stderr from parent).
 	 * - Specify -1 to have a pipe allocated as follows:
 	 *     .in: returns the writable pipe end; parent writes to it,
 	 *          the readable pipe end becomes child's stdin
@@ -37,13 +89,43 @@ struct child_process {
 	int in;
 	int out;
 	int err;
+
+	/**
+	 * To specify a new initial working directory for the sub-process,
+	 * specify it in the .dir member.
+	 */
 	const char *dir;
+
+	/**
+	 * To modify the environment of the sub-process, specify an array of
+	 * string pointers (NULL terminated) in .env:
+	 *
+	 * - If the string is of the form "VAR=value", i.e. it contains '='
+	 *   the variable is added to the child process's environment.
+	 *
+	 * - If the string does not contain '=', it names an environment
+	 *   variable that will be removed from the child process's environment.
+	 *
+	 * If the .env member is NULL, `start_command` will point it at the
+	 * .env_array `argv_array` (so you may use one or the other, but not both).
+	 * The memory in .env_array will be cleaned up automatically during
+	 * `finish_command` (or during `start_command` when it is unsuccessful).
+	 */
 	const char *const *env;
+
 	unsigned no_stdin:1;
 	unsigned no_stdout:1;
 	unsigned no_stderr:1;
 	unsigned git_cmd:1; /* if this is to be git sub-command */
+
+	/**
+	 * If the program cannot be found, the functions return -1 and set
+	 * errno to ENOENT. Normally, an error message is printed, but if
+	 * .silent_exec_failure is set to 1, no message is printed for this
+	 * special error condition.
+	 */
 	unsigned silent_exec_failure:1;
+
 	unsigned stdout_to_stderr:1;
 	unsigned use_shell:1;
 	unsigned clean_on_exit:1;
@@ -53,13 +135,63 @@ struct child_process {
 };
 
 #define CHILD_PROCESS_INIT { NULL, ARGV_ARRAY_INIT, ARGV_ARRAY_INIT }
+
+/**
+ * The functions: child_process_init, start_command, finish_command,
+ * run_command, run_command_v_opt, run_command_v_opt_cd_env, child_process_clear
+ * do the following:
+ *
+ * - If a system call failed, errno is set and -1 is returned. A diagnostic
+ *   is printed.
+ *
+ * - If the program was not found, then -1 is returned and errno is set to
+ *   ENOENT; a diagnostic is printed only if .silent_exec_failure is 0.
+ *
+ * - Otherwise, the program is run. If it terminates regularly, its exit
+ *   code is returned. No diagnostic is printed, even if the exit code is
+ *   non-zero.
+ *
+ * - If the program terminated due to a signal, then the return value is the
+ *   signal number + 128, ie. the same value that a POSIX shell's $? would
+ *   report.  A diagnostic is printed.
+ *
+ */
+
+/**
+ * Initialize a struct child_process variable.
+ */
 void child_process_init(struct child_process *);
+
+/**
+ * Release the memory associated with the struct child_process.
+ * Most users of the run-command API don't need to call this
+ * function explicitly because `start_command` invokes it on
+ * failure and `finish_command` calls it automatically already.
+ */
 void child_process_clear(struct child_process *);
+
 int is_executable(const char *name);
 
+/**
+ * Start a sub-process. Takes a pointer to a `struct child_process`
+ * that specifies the details and returns pipe FDs (if requested).
+ * See below for details.
+ */
 int start_command(struct child_process *);
+
+/**
+ * Wait for the completion of a sub-process that was started with
+ * start_command().
+ */
 int finish_command(struct child_process *);
+
 int finish_command_in_signal(struct child_process *);
+
+/**
+ * A convenience function that encapsulates a sequence of
+ * start_command() followed by finish_command(). Takes a pointer
+ * to a `struct child_process` that specifies the details.
+ */
 int run_command(struct child_process *);
 
 /*
@@ -68,6 +200,20 @@ int run_command(struct child_process *);
  * overwritten by further calls to find_hook and run_hook_*.
  */
 const char *find_hook(const char *name);
+
+/**
+ * Run a hook.
+ * The first argument is a pathname to an index file, or NULL
+ * if the hook uses the default index file or no index is needed.
+ * The second argument is the name of the hook.
+ * The further arguments correspond to the hook arguments.
+ * The last argument has to be NULL to terminate the arguments list.
+ * If the hook does not exist or is not executable, the return
+ * value will be zero.
+ * If it is executable, the hook will be executed and the exit
+ * status of the hook is returned.
+ * On execution, .stdout_to_stderr and .no_stdin will be set.
+ */
 LAST_ARG_MUST_BE_NULL
 int run_hook_le(const char *const *env, const char *name, ...);
 int run_hook_ve(const char *const *env, const char *name, va_list args);
@@ -78,6 +224,18 @@ int run_hook_ve(const char *const *env, const char *name, va_list args);
 #define RUN_SILENT_EXEC_FAILURE 8
 #define RUN_USING_SHELL 16
 #define RUN_CLEAN_ON_EXIT 32
+
+/**
+ * Convenience functions that encapsulate a sequence of
+ * start_command() followed by finish_command(). The argument argv
+ * specifies the program and its arguments. The argument opt is zero
+ * or more of the flags `RUN_COMMAND_NO_STDIN`, `RUN_GIT_CMD`,
+ * `RUN_COMMAND_STDOUT_TO_STDERR`, or `RUN_SILENT_EXEC_FAILURE`
+ * that correspond to the members .no_stdin, .git_cmd,
+ * .stdout_to_stderr, .silent_exec_failure of `struct child_process`.
+ * The argument dir corresponds the member .dir. The argument env
+ * corresponds to the member .env.
+ */
 int run_command_v_opt(const char **argv, int opt);
 int run_command_v_opt_tr2(const char **argv, int opt, const char *tr2_class);
 /*
@@ -125,15 +283,84 @@ static inline int capture_command(struct child_process *cmd,
  * It is expected that no synchronization and mutual exclusion between
  * the caller and the feed function is necessary so that the function
  * can run in a thread without interfering with the caller.
+ *
+ * The caller:
+ *
+ * 1. allocates and clears (memset(&asy, 0, sizeof(asy));) a
+ *    struct async variable;
+ * 2. initializes .proc and .data;
+ * 3. calls start_async();
+ * 4. processes communicates with proc through .in and .out;
+ * 5. closes .in and .out;
+ * 6. calls finish_async().
+ *
+ * There are serious restrictions on what the asynchronous function can do
+ * because this facility is implemented by a thread in the same address
+ * space on most platforms (when pthreads is available), but by a pipe to
+ * a forked process otherwise:
+ *
+ * - It cannot change the program's state (global variables, environment,
+ *   etc.) in a way that the caller notices; in other words, .in and .out
+ *   are the only communication channels to the caller.
+ *
+ * - It must not change the program's state that the caller of the
+ *   facility also uses.
+ *
  */
 struct async {
-	/*
-	 * proc reads from in; closes it before return
-	 * proc writes to out; closes it before return
-	 * returns 0 on success, non-zero on failure
+
+	/**
+	 * The function pointer in .proc has the following signature:
+	 *
+	 *	int proc(int in, int out, void *data);
+	 *
+	 * - in, out specifies a set of file descriptors to which the function
+	 *  must read/write the data that it needs/produces.  The function
+	 *  *must* close these descriptors before it returns.  A descriptor
+	 *  may be -1 if the caller did not configure a descriptor for that
+	 *  direction.
+	 *
+	 * - data is the value that the caller has specified in the .data member
+	 *  of struct async.
+	 *
+	 * - The return value of the function is 0 on success and non-zero
+	 *  on failure. If the function indicates failure, finish_async() will
+	 *  report failure as well.
+	 *
 	 */
 	int (*proc)(int in, int out, void *data);
+
 	void *data;
+
+	/**
+	 * The members .in, .out are used to provide a set of fd's for
+	 * communication between the caller and the callee as follows:
+	 *
+	 * - Specify 0 to have no file descriptor passed.  The callee will
+	 *   receive -1 in the corresponding argument.
+	 *
+	 * - Specify < 0 to have a pipe allocated; start_async() replaces
+	 *   with the pipe FD in the following way:
+	 *
+	 * 	.in: Returns the writable pipe end into which the caller
+	 * 	writes; the readable end of the pipe becomes the function's
+	 * 	in argument.
+	 *
+	 * 	.out: Returns the readable pipe end from which the caller
+	 * 	reads; the writable end of the pipe becomes the function's
+	 * 	out argument.
+	 *
+	 *   The caller of start_async() must close the returned FDs after it
+	 *   has completed reading from/writing from them.
+	 *
+	 * - Specify a file descriptor > 0 to be used by the function:
+	 *
+	 * 	.in: The FD must be readable; it becomes the function's in.
+	 * 	.out: The FD must be writable; it becomes the function's out.
+	 *
+	 *   The specified FD is closed by start_async(), even if it fails to
+	 *   run the function.
+	 */
 	int in;		/* caller writes here and closes it */
 	int out;	/* caller reads from here and closes it */
 #ifdef NO_PTHREADS
@@ -146,8 +373,19 @@ struct async {
 	int isolate_sigpipe;
 };
 
+/**
+ * Run a function asynchronously. Takes a pointer to a `struct
+ * async` that specifies the details and returns a set of pipe FDs
+ * for communication with the function. See below for details.
+ */
 int start_async(struct async *async);
+
+/**
+ * Wait for the completion of an asynchronous function that was
+ * started with start_async().
+ */
 int finish_async(struct async *async);
+
 int in_async(void);
 int async_with_fork(void);
 void check_pipe(int err);