doc: describe when stdout/err is sync

process.stdout, process.stderr, and console.log() and console.error()
which use the process streams, are usually synchronous. Warn about this,
and clearly describe the conditions under which they are synchronous.

Fix: #10617
PR-URL: #10884
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>

doc/api/console.md

@@ -9,10 +9,15 @@ The module exports two specific components:
 
 * A `Console` class with methods such as `console.log()`, `console.error()` and
   `console.warn()` that can be used to write to any Node.js stream.
-* A global `console` instance configured to write to `stdout` and `stderr`.
-  Because this object is global, it can be used without calling
+* A global `console` instance configured to write to [`process.stdout`][] and
+  [`process.stderr`][].  The global `console` can be used without calling
   `require('console')`.
 
+***Warning***: The global console object's methods are neither consistently
+synchronous like the browser APIs they resemble, nor are they consistently
+asynchronous like all other Node.js streams. See the [note on process I/O][] for
+more information.
+
 Example using the global `console`:
 
 ```js
@@ -47,21 +52,6 @@ myConsole.warn(`Danger ${name}! Danger!`);
 // Prints: Danger Will Robinson! Danger!, to err
 ```
 
-While the API for the `Console` class is designed fundamentally around the
-browser `console` object, the `Console` in Node.js is *not* intended to
-duplicate the browser's functionality exactly.
-
-## Asynchronous vs Synchronous Consoles
-
-The console functions are usually asynchronous unless the destination is a file.
-Disks are fast and operating systems normally employ write-back caching;
-it should be a very rare occurrence indeed that a write blocks, but it
-is possible.
-
-Additionally, console functions are blocking when outputting to TTYs
-(terminals) on OS X as a workaround for the OS's very small, 1kb buffer size.
-This is to prevent interleaving between `stdout` and `stderr`.
-
 ## Class: Console
 
 <!--type=class-->
@@ -305,4 +295,5 @@ The `console.warn()` function is an alias for [`console.error()`][].
 [`util.format()`]: util.html#util_util_format_format_args
 [`util.inspect()`]: util.html#util_util_inspect_object_options
 [customizing `util.inspect()` colors]: util.html#util_customizing_util_inspect_colors
+[note on process I/O]: process.html#process_a_note_on_process_i_o
 [web-api-assert]: https://developer.mozilla.org/en-US/docs/Web/API/console/assert

doc/api/process.md

@@ -850,10 +850,11 @@ added: v0.1.13
 
 * `code` {Integer} The exit code. Defaults to `0`.
 
-The `process.exit()` method instructs Node.js to terminate the process as
-quickly as possible with the specified exit `code`. If the `code` is omitted,
-exit uses either the 'success' code `0` or the value of `process.exitCode` if
-specified.
+The `process.exit()` method instructs Node.js to terminate the process
+synchronously with an exit status of `code`. If `code` is omitted, exit uses
+either the 'success' code `0` or the value of `process.exitCode` if it has been
+set.  Node.js will not terminate until all the [`'exit'`] event listeners are
+called.
 
 To exit with a 'failure' code:
 
@@ -886,7 +887,7 @@ if (someConditionNotMet()) {
 ```
 
 The reason this is problematic is because writes to `process.stdout` in Node.js
-are sometimes *non-blocking* and may occur over multiple ticks of the Node.js
+are sometimes *asynchronous* and may occur over multiple ticks of the Node.js
 event loop. Calling `process.exit()`, however, forces the process to exit
 *before* those additional writes to `stdout` can be performed.
 
@@ -1475,23 +1476,11 @@ Android)
 
 * {Stream}
 
-The `process.stderr` property returns a [Writable][] stream equivalent to or
-associated with `stderr` (fd `2`).
+The `process.stderr` property returns a [Writable][] stream connected to
+`stderr` (fd `2`).
 
-Note: `process.stderr` and `process.stdout` differ from other Node.js streams
-in several ways:
-1. They cannot be closed ([`end()`][] will throw).
-2. They never emit the [`'finish'`][] event.
-3. Writes _can_ block when output is redirected to a file.
-  - Note that disks are fast and operating systems normally employ write-back
-    caching so this is very uncommon.
-4. Writes on UNIX **will** block by default if output is going to a TTY
-   (a terminal).
-5. Windows functionality differs. Writes block except when output is going to a
-   TTY.
-
-To check if Node.js is being run in a TTY context, read the `isTTY` property
-on `process.stderr`, `process.stdout`, or `process.stdin`:
+Note: `process.stderr` differs from other Node.js streams in important ways,
+see [note on process I/O][] for more information.
 
 ## process.stdin
 
@@ -1529,48 +1518,59 @@ must call `process.stdin.resume()` to read from it. Note also that calling
 
 * {Stream}
 
-The `process.stdout` property returns a [Writable][] stream equivalent to or
-associated with `stdout` (fd `1`).
+The `process.stdout` property returns a [Writable][] stream connected to
+`stdout` (fd `2`).
 
-For example:
+For example, to copy process.stdin to process.stdout:
 
 ```js
-console.log = (msg) => {
-  process.stdout.write(`${msg}\n`);
-};
+process.stdin.pipe(process.stdout);
 ```
 
-Note: `process.stderr` and `process.stdout` differ from other Node.js streams
-in several ways:
-1. They cannot be closed ([`end()`][] will throw).
-2. They never emit the [`'finish'`][] event.
-3. Writes _can_ block when output is redirected to a file.
-  - Note that disks are fast and operating systems normally employ write-back
-    caching so this is very uncommon.
-4. Writes on UNIX **will** block by default if output is going to a TTY
-   (a terminal).
-5. Windows functionality differs. Writes block except when output is going to a
-   TTY.
+Note: `process.stdout` differs from other Node.js streams in important ways,
+see [note on process I/O][] for more information.
+
+### A note on process I/O
 
-To check if Node.js is being run in a TTY context, read the `isTTY` property
-on `process.stderr`, `process.stdout`, or `process.stdin`:
+`process.stdout` and `process.stderr` differ from other Node.js streams in
+important ways:
 
-### TTY Terminals and `process.stdout`
+1. They are used internally by [`console.log()`][] and [`console.error()`][],
+   respectively.
+2. They cannot be closed ([`end()`][] will throw).
+3. They will never emit the [`'finish'`][] event.
+4. Writes may be synchronous depending on the what the stream is connected to
+   and whether the system is Windows or Unix:
+   - Files: *synchronous* on Windows and Linux
+   - TTYs (Terminals): *asynchronous* on Windows, *synchronous* on Unix
+   - Pipes (and sockets): *synchronous* on Windows, *asynchronous* on Unix
 
-The `process.stderr` and `process.stdout` streams are blocking when outputting
-to TTYs (terminals) on OS X as a workaround for the operating system's small,
-1kb buffer size. This is to prevent interleaving between `stdout` and `stderr`.
+These behaviours are partly for historical reasons, as changing them would
+create backwards incompatibility, but they are also expected by some users.
 
-To check if Node.js is being run in a [TTY][] context, check the `isTTY`
-property on `process.stderr`, `process.stdout`, or `process.stdin`.
+Synchronous writes avoid problems such as output written with `console.log()` or
+`console.write()` being unexpectedly interleaved, or not written at all if
+`process.exit()` is called before an asynchronous write completes. See
+[`process.exit()`][] for more information.
+
+***Warning***: Synchronous writes block the event loop until the write has
+completed. This can be near instantaneous in the case of output to a file, but
+under high system load, pipes that are not being read at the receiving end, or
+with slow terminals or file systems, its possible for the event loop to be
+blocked often enough and long enough to have severe negative performance
+impacts. This may not be a problem when writing to an interactive terminal
+session, but consider this particularly careful when doing production logging to
+the process output streams.
+
+To check if a stream is connected to a [TTY][] context, check the `isTTY`
+property.
 
 For instance:
 ```console
 $ node -p "Boolean(process.stdin.isTTY)"
 true
 $ echo "foo" | node -p "Boolean(process.stdin.isTTY)"
 false
-
 $ node -p "Boolean(process.stdout.isTTY)"
 true
 $ node -p "Boolean(process.stdout.isTTY)" | cat
@@ -1724,6 +1724,7 @@ cases:
   the high-order bit, and then contain the value of the signal code.
 
 
+[`'exit'`]: #process_event_exit
 [`'finish'`]: stream.html#stream_event_finish
 [`'message'`]: child_process.html#child_process_event_message
 [`'rejectionHandled'`]: #process_event_rejectionhandled
@@ -1745,6 +1746,7 @@ cases:
 [`promise.catch()`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise/catch
 [`require.main`]: modules.html#modules_accessing_the_main_module
 [`setTimeout(fn, 0)`]: timers.html#timers_settimeout_callback_delay_args
+[note on process I/O]: process.html#process_a_note_on_process_i_o
 [process_emit_warning]: #process_process_emitwarning_warning_name_ctor
 [process_warning]: #process_event_warning
 [Signal Events]: #process_signal_events