fs: support pseudofiles in promises.readFile

PR-URL: #21497Fixes: #21331
Refs: http://pubs.opengroup.org/onlinepubs/9699919799/functions/read.html
Refs: https://groups.google.com/forum/#!topic/nodejs-dev/rxZ_RoH1Gn0
Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>

Author: TimothyGu

lib/internal/fs/promises.js

@@ -125,6 +125,10 @@ async function writeFileHandle(filehandle, data, options) {
 }while(remaining>0);
 }
 
+// Note: This is different from kReadFileBufferLength used for non-promisified
+// fs.readFile.
+constkReadFileMaxChunkSize=16384;
+
 asyncfunctionreadFileHandle(filehandle,options){
 conststatFields=awaitbinding.fstat(filehandle.fd,false,kUsePromises);
 
@@ -135,22 +139,19 @@ async function readFileHandle(filehandle, options) {
 size=0;
 }
 
-if(size===0)
-returnoptions.encoding ? '' : Buffer.alloc(0);
-
 if(size>kMaxLength)
 thrownewERR_FS_FILE_TOO_LARGE(size);
 
 constchunks=[];
-constchunkSize=Math.min(size,16384);
-lettotalRead=0;
+constchunkSize=size===0 ?
+kReadFileMaxChunkSize :
+Math.min(size,kReadFileMaxChunkSize);
 letendOfFile=false;
 do{
 constbuf=Buffer.alloc(chunkSize);
 const{ bytesRead, buffer }=
-awaitread(filehandle,buf,0,chunkSize,totalRead);
-totalRead+=bytesRead;
-endOfFile=bytesRead!==chunkSize;
+awaitread(filehandle,buf,0,chunkSize,-1);
+endOfFile=bytesRead===0;
 if(bytesRead>0)
 chunks.push(buffer.slice(0,bytesRead));
 }while(!endOfFile);

test/parallel/test-fs-promises-file-handle-readFile.js

@@ -28,5 +28,22 @@ async function validateReadFile() {
 assert.deepStrictEqual(buffer,readFileData);
 }
 
+asyncfunctionvalidateReadFileProc(){
+// Test to make sure reading a file under the /proc directory works. Adapted
+// from test-fs-read-file-sync-hostname.js.
+// Refs:
+// - https://groups.google.com/forum/#!topic/nodejs-dev/rxZ_RoH1Gn0
+// - https://github.com/nodejs/node/issues/21331
+
+// Test is Linux-specific.
+if(!common.isLinux)
+return;
+
+constfileHandle=awaitopen('/proc/sys/kernel/hostname','r');
+consthostname=awaitfileHandle.readFile();
+assert.ok(hostname.length>0);
+}
+
 validateReadFile()
+.then(()=>validateReadFileProc())
 .then(common.mustCall());

test/parallel/test-fs-promises-readfile.js

@@ -12,17 +12,35 @@ const fn = path.join(tmpdir.path, 'large-file');
 
 common.crashOnUnhandledRejection();
 
-// Creating large buffer with random content
-constbuffer=Buffer.from(
-Array.apply(null,{length: 16834*2})
-.map(Math.random)
-.map((number)=>(number*(1<<8)))
-);
-
-// Writing buffer to a file then try to read it
-writeFile(fn,buffer)
-.then(()=>readFile(fn))
-.then((readBuffer)=>{
-assert.strictEqual(readBuffer.equals(buffer),true);
-})
+asyncfunctionvalidateReadFile(){
+// Creating large buffer with random content
+constbuffer=Buffer.from(
+Array.apply(null,{length: 16834*2})
+.map(Math.random)
+.map((number)=>(number*(1<<8)))
+);
+
+// Writing buffer to a file then try to read it
+awaitwriteFile(fn,buffer);
+constreadBuffer=awaitreadFile(fn);
+assert.strictEqual(readBuffer.equals(buffer),true);
+}
+
+asyncfunctionvalidateReadFileProc(){
+// Test to make sure reading a file under the /proc directory works. Adapted
+// from test-fs-read-file-sync-hostname.js.
+// Refs:
+// - https://groups.google.com/forum/#!topic/nodejs-dev/rxZ_RoH1Gn0
+// - https://github.com/nodejs/node/issues/21331
+
+// Test is Linux-specific.
+if(!common.isLinux)
+return;
+
+consthostname=awaitreadFile('/proc/sys/kernel/hostname');
+assert.ok(hostname.length>0);
+}
+
+validateReadFile()
+.then(()=>validateReadFileProc())
 .then(common.mustCall());