process: add --redirect-warnings command line argument

The --redirect-warnings command line argument allows process warnings
to be written to a specified file rather than printed to stderr.

Also adds an equivalent NODE_REDIRECT_WARNINGS environment variable.

If the specified file cannot be opened or written to for any reason,
the argument is ignored and the warning is printed to stderr.

If the file already exists, it will be appended to.

Author: jasnell

doc/api/cli.md

@@ -121,6 +121,16 @@ added: v6.0.0
 
 Print stack traces for process warnings (including deprecations).
 
+### `--redirect-warnings=file`
+<!-- YAML
+added: REPLACEME
+-->
+
+Write process warnings to the given file instead of printing to stderr. The
+file will be created if it does not exist, and will be appended to if it does.
+If an error occurs while attempting to write the warning to the file, the
+warning will be written to stderr instead.
+
 ### `--trace-sync-io`
 <!-- YAML
 added: v2.1.0
@@ -395,6 +405,17 @@ Note: Be aware that unless the child environment is explicitly set, this
 evironment variable will be inherited by any child processes, and if they use
 OpenSSL, it may cause them to trust the same CAs as node.
 
+### `NODE_REDIRECT_WARNINGS=file`
+<!-- YAML
+added: REPLACEME
+-->
+
+When set, process warnings will be emitted to the given file instead of
+printing to stderr. The file will be created if it does not exist, and will be
+appended to if it does. If an error occurs while attempting to write the
+warning to the file, the warning will be written to stderr instead. This is
+equivalent to using the `--redirect-warnings=file` command-line flag.
+
 [emit_warning]: process.html#process_process_emitwarning_warning_name_ctor
 [Buffer]: buffer.html#buffer_buffer
 [debugger]: debugger.html

doc/node.1

@@ -112,6 +112,10 @@ Silence all process warnings (including deprecations).
 .BR \-\-trace\-warnings
 Print stack traces for process warnings (including deprecations).
 
+.TP
+.BR \-\-redirect\-warnings=\fIfile\fR
+Write process warnings to the given file instead of printing to stderr.
+
 .TP
 .BR \-\-trace\-sync\-io
 Print a stack trace whenever synchronous I/O is detected after the first turn
@@ -262,6 +266,12 @@ containing trusted certificates.
 If \fB\-\-use\-openssl\-ca\fR is enabled, this overrides and sets OpenSSL's
 file containing trusted certificates.
 
+.TP
+.BR NODE_REDIRECT_WARNINGS=\fIfile\fR
+Write process warnings to the given file instead of printing to stderr.
+(equivalent to using the \-\-redirect\-warnings=\fIfile\fR command-line
+argument).
+
 .SH BUGS
 Bugs are tracked in GitHub Issues:
 .ur https://github.com/nodejs/node/issues

lib/internal/process/warning.js

@@ -1,9 +1,80 @@
 'use strict';
 
+const config = process.binding('config');
 const prefix = `(${process.release.name}:${process.pid}) `;
 
 exports.setup = setupProcessWarnings;
 
+var fs;
+var cachedFd;
+var acquiringFd = false;
+function nop() {}
+
+function lazyFs() {
+  if (!fs)
+    fs = require('fs');
+  return fs;
+}
+
+function writeOut(message) {
+  if (console && typeof console.error === 'function')
+    return console.error(message);
+  process._rawDebug(message);
+}
+
+function onClose(fd) {
+  return function() {
+    lazyFs().close(fd, nop);
+  };
+}
+
+function onOpen(cb) {
+  return function(err, fd) {
+    acquiringFd = false;
+    if (fd !== undefined) {
+      cachedFd = fd;
+      process.on('exit', onClose(fd));
+    }
+    cb(err, fd);
+    process.emit('_node_warning_fd_acquired', err, fd);
+  };
+}
+
+function onAcquired(message) {
+  // make a best effort attempt at writing the message
+  // to the fd. Errors are ignored at this point.
+  return function(err, fd) {
+    if (err)
+      return writeOut(message);
+    lazyFs().appendFile(fd, `${message}\n`, nop);
+  };
+}
+
+function acquireFd(cb) {
+  if (cachedFd === undefined && !acquiringFd) {
+    acquiringFd = true;
+    lazyFs().open(config.warningFile, 'a', onOpen(cb));
+  } else if (cachedFd !== undefined && !acquiringFd) {
+    cb(null, cachedFd);
+  } else {
+    process.once('_node_warning_fd_acquired', cb);
+  }
+}
+
+function output(message) {
+  if (typeof config.warningFile === 'string') {
+    acquireFd(onAcquired(message));
+    return;
+  }
+  writeOut(message);
+}
+
+function doEmitWarning(warning) {
+  return function() {
+    process.emit('warning', warning);
+  };
+}
+
 function setupProcessWarnings() {
   if (!process.noProcessWarnings && process.env.NODE_NO_WARNINGS !== '1') {
     process.on('warning', (warning) => {
@@ -14,19 +85,18 @@ function setupProcessWarnings() {
                     (isDeprecation && process.traceDeprecation);
       if (trace && warning.stack) {
         if (warning.code) {
-          console.error(`${prefix}[${warning.code}] ${warning.stack}`);
+          output(`${prefix}[${warning.code}] ${warning.stack}`);
         } else {
-          console.error(`${prefix}${warning.stack}`);
+          output(`${prefix}${warning.stack}`);
         }
       } else {
         const toString =
           typeof warning.toString === 'function' ?
             warning.toString : Error.prototype.toString;
         if (warning.code) {
-          console.error(
-              `${prefix}[${warning.code}] ${toString.apply(warning)}`);
+          output(`${prefix}[${warning.code}] ${toString.apply(warning)}`);
         } else {
-          console.error(`${prefix}${toString.apply(warning)}`);
+          output(`${prefix}${toString.apply(warning)}`);
         }
       }
     });
@@ -63,6 +133,6 @@ function setupProcessWarnings() {
       if (process.throwDeprecation)
         throw warning;
     }
-    process.nextTick(() => process.emit('warning', warning));
+    process.nextTick(doEmitWarning(warning));
   };
 }

src/node.cc

@@ -188,6 +188,9 @@ bool trace_warnings = false;
 // that is used by lib/module.js
 bool config_preserve_symlinks = false;
 
+// Set in node.cc by ParseArgs when --redirect-warnings= is used.
+const char* config_warning_file;
+
 bool v8_initialized = false;
 
 // process-relative uptime base, initialized at start-up
@@ -3499,6 +3502,9 @@ static void PrintHelp() {
          "  --throw-deprecation        throw an exception on deprecations\n"
          "  --no-warnings              silence all process warnings\n"
          "  --trace-warnings           show stack traces on process warnings\n"
+         "  --redirect-warnings=path\n"
+         "                             write warnings to path instead of\n"
+         "                             stderr\n"
          "  --trace-sync-io            show stack trace when use of sync IO\n"
          "                             is detected after the first tick\n"
          "  --trace-events-enabled     track trace events\n"
@@ -3564,6 +3570,8 @@ static void PrintHelp() {
          "                             prefixed to the module search path\n"
          "NODE_REPL_HISTORY            path to the persistent REPL history\n"
          "                             file\n"
+         "NODE_REDIRECT_WARNINGS       write warnings to path instead of\n"
+         "                             stderr\n"
          "Documentation can be found at https://nodejs.org/\n");
 }
 
@@ -3664,6 +3672,8 @@ static void ParseArgs(int* argc,
       no_process_warnings = true;
     } else if (strcmp(arg, "--trace-warnings") == 0) {
       trace_warnings = true;
+    } else if (strncmp(arg, "--redirect-warnings=", 20) == 0) {
+      config_warning_file = arg + 20;
     } else if (strcmp(arg, "--trace-deprecation") == 0) {
       trace_deprecation = true;
     } else if (strcmp(arg, "--trace-sync-io") == 0) {
@@ -4206,6 +4216,10 @@ void Init(int* argc,
     config_preserve_symlinks = (*preserve_symlinks == '1');
   }
 
+  if (auto redirect_warnings = secure_getenv("NODE_REDIRECT_WARNINGS")) {
+    config_warning_file = redirect_warnings;
+  }
+
   // Parse a few arguments which are specific to Node.
   int v8_argc;
   const char** v8_argv;

src/node_config.cc

@@ -12,6 +12,7 @@ using v8::Context;
 using v8::Local;
 using v8::Object;
 using v8::ReadOnly;
+using v8::String;
 using v8::Value;
 
 // The config binding is used to provide an internal view of compile or runtime
@@ -44,6 +45,15 @@ void InitConfig(Local<Object> target,
 
   if (config_preserve_symlinks)
     READONLY_BOOLEAN_PROPERTY("preserveSymlinks");
+
+  if (config_warning_file != nullptr) {
+    Local<String> name = OneByteString(env->isolate(), "warningFile");
+    Local<String> value = String::NewFromUtf8(env->isolate(),
+                                              config_warning_file,
+                                              v8::NewStringType::kNormal)
+                                                .ToLocalChecked();
+    target->DefineOwnProperty(env->context(), name, value).FromJust();
+  }
 }  // InitConfig
 
 }  // namespace node

src/node_internals.h

@@ -42,6 +42,11 @@ extern const char* openssl_config;
 // that is used by lib/module.js
 extern bool config_preserve_symlinks;
 
+// Set in node.cc by ParseArgs when --redirect-warnings= is used.
+// Used to redirect warning output to a file rather than sending
+// it to stderr.
+extern const char* config_warning_file;
+
 // Tells whether it is safe to call v8::Isolate::GetCurrent().
 extern bool v8_initialized;
 

test/parallel/test-process-redirect-warnings-env.js

@@ -0,0 +1,25 @@
+'use strict';
+
+// Tests the NODE_REDIRECT_WARNINGS environment variable by spawning
+// a new child node process that emits a warning into a temporary
+// warnings file. Once the process completes, the warning file is
+// opened and the contents are validated
+
+const common = require('../common');
+const fs = require('fs');
+const fork = require('child_process').fork;
+const path = require('path');
+const assert = require('assert');
+
+common.refreshTmpDir();
+
+const warnmod = require.resolve(common.fixturesDir + '/warnings.js');
+const warnpath = path.join(common.tmpDir, 'warnings.txt');
+
+fork(warnmod, {env: {NODE_REDIRECT_WARNINGS: warnpath}})
+  .on('exit', common.mustCall(() => {
+    fs.readFile(warnpath, 'utf8', common.mustCall((err, data) => {
+      assert.ifError(err);
+      assert(/\(node:\d+\) Warning: a bad practice warning/.test(data));
+    }));
+  }));

test/parallel/test-process-redirect-warnings.js

@@ -0,0 +1,25 @@
+'use strict';
+
+// Tests the --redirect-warnings command line flag by spawning
+// a new child node process that emits a warning into a temporary
+// warnings file. Once the process completes, the warning file is
+// opened and the contents are validated
+
+const common = require('../common');
+const fs = require('fs');
+const fork = require('child_process').fork;
+const path = require('path');
+const assert = require('assert');
+
+common.refreshTmpDir();
+
+const warnmod = require.resolve(common.fixturesDir + '/warnings.js');
+const warnpath = path.join(common.tmpDir, 'warnings.txt');
+
+fork(warnmod, {execArgv: [`--redirect-warnings=${warnpath}`]})
+  .on('exit', common.mustCall(() => {
+    fs.readFile(warnpath, 'utf8', common.mustCall((err, data) => {
+      assert.ifError(err);
+      assert(/\(node:\d+\) Warning: a bad practice warning/.test(data));
+    }));
+  }));