buggy.js

// for some reason from time to time it may output WTFs
// the way i run it: trace -f node buggy.js 2>&1 | grep '\(pwrite\|pread\|WTF\)'
// all reads come after writes

var fs   = require("fs"),
	file = fs.openSync("aa", "a+"),
	bufs = {},
	off  = fs.fstatSync(file).size,
	offs = {};
	
offs = { '4890': 140278,
  '7928': 51093,
  '8731': 0,
  '11806': 59021,
  '14394': 125884,
  '17800': 108084,
  '18028': 70827,
  '19229': 88855,
  '21116': 29977,
  '21246': 8731 }
  
Object.keys(offs).forEach(function(o) {
	offs[o] += off;
});

function buf(size) {
	if (!bufs[size]) {
		bufs[size] = new Buffer(size);
	}
	
	return bufs[size];
}

function write(size, callback) {
	var offset = off;
	offs[size] = offset;
	off += size;
	fs.write(file, buf(size), 0, buf(size).length, offset, callback);
}

function reader() {
	console.log(bufs);
	console.log(offs)
	Object.keys(bufs).forEach(function(size) {
		size = +size;
		var offset = offs[size],
			test   = new Buffer(size);
		fs.read(file, test, 0, test.length, offset, function() {
			if (test.toString("base64") != buf(size).toString("base64")) {
				console.log("WTF? " + test.length + ":" + buf(size).length);
				console.log(buf(size).slice(0,4).toString("base64"), test.slice(0, 4).toString("base64"))
			}
		});
	});
}

write(8731)
write(21246)
write(21116)
write(7928)
write(11806)

write(18028)
write(19229)
write(17800)
write(14394)
write(4890)

reader()

strace.txt

# strace -f node buggy.js 2>&1 | grep '\(pwrite\|pread\|WTF\)'
[pid 12081] pread(7, "var fs   = require(\"fs\"),\n\tfile "..., 1775, 0) = 1775
[pid 12083] pwrite(7, "H\213T$ \366\302\1\17\204a\1\0\0H\203\354 H\211T$\10I\272\20\312\200\1\0\0\0"..., 8731, 3193696 <unfinished ...>
[pid 12083] <... pwrite resumed> )      = 8731
[pid 12083] pwrite(7, "\377\377\377\377\0\0\0\0\0\0\0\0\0\0\0\0\1\0\1\1\1\0\0\0\260\221:\315\267\177\0\0"..., 21246, 3202427 <unfinished ...>
[pid 12083] <... pwrite resumed> )      = 21246
[pid 12083] pwrite(7, "\0\0\0\0\2\0\0\0\0\0\0\0\377\377\377\377\377\377\377\377\0\0\0\0\0\0\0\0\0\0\0\0"..., 21116, 3223673 <unfinished ...>
[pid 12083] <... pwrite resumed> )      = 21116
[pid 12083] pwrite(7, "\230\305\275\313\267\177\0\0\230\305\275\313\267\177\0\0\240\302\201\1\0\0\0\0\240\302\201\1\0\0\0\0"..., 7928, 3244789 <unfinished ...>
[pid 12083] <... pwrite resumed> )      = 7928
[pid 12083] pwrite(7, "0\244:\315\267\177\0\0\277\0\0\0\300\0\0\0005\0\0\0\0\0\0\0\220+\203\1\0\0\0\0"..., 11806, 3252717 <unfinished ...>
[pid 12083] <... pwrite resumed> )      = 11806
[pid 12083] pwrite(7, "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 19229, 3282551 <unfinished ...>
[pid 12083] <... pwrite resumed> )      = 19229
[pid 12084] pwrite(7, "\0\244|\1\0\0\0\0\0\0\0\0\0\0\0\0Xz\203\1\0\0\0\0C\0\0\0*\0\0\0"..., 18028, 3264523 <unfinished ...>
[pid 12084] <... pwrite resumed> )      = 18028
[pid 12083] pwrite(7, "\360\233:\315\267\177\0\0\31\0\0\0\32\0\0\0 \336\200\1\0\0\0\0\230\324\203\1\0\0\0\0"..., 17800, 3301780 <unfinished ...>
[pid 12083] <... pwrite resumed> )      = 17800
[pid 12083] pwrite(7, "\260\222:\315\267\177\0\0\377\377\377\377\0\0\0\0000\223:\315\267\177\0\0\241)\0\0\0\0\0\0"..., 14394, 3319580 <unfinished ...>
[pid 12083] <... pwrite resumed> )      = 14394
[pid 12085] pwrite(7, "p\303|\1\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\244|\1\0\0\0\0"..., 4890, 3333974) = 4890
[pid 12084] pread(7,  <unfinished ...>
[pid 12084] <... pread resumed> "p\303|\1\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\244|\1\0\0\0\0"..., 4890, 3333974) = 4890
[pid 12084] pread(7,  <unfinished ...>
[pid 12084] <... pread resumed> "\230\305\275\313\267\177\0\0\230\305\275\313\267\177\0\0\240\302\201\1\0\0\0\0\240\302\201\1\0\0\0\0"..., 7928, 3244789) = 7928
[pid 12085] pread(7,  <unfinished ...>
[pid 12085] <... pread resumed> "H\213T$ \366\302\1\17\204a\1\0\0H\203\354 H\211T$\10I\272\20\312\200\1\0\0\0"..., 8731, 3193696) = 8731
[pid 12085] pread(7,  <unfinished ...>
[pid 12085] <... pread resumed> "0\244:\315\267\177\0\0\277\0\0\0\300\0\0\0005\0\0\0\0\0\0\0\220+\203\1\0\0\0\0"..., 11806, 3252717) = 11806
[pid 12084] pread(7,  <unfinished ...>
[pid 12084] <... pread resumed> "\260\222:\315\267\177\0\0\377\377\377\377\0\0\0\0000\223:\315\267\177\0\0\241)\0\0\0\0\0\0"..., 14394, 3319580) = 14394
[pid 12085] pread(7,  <unfinished ...>
[pid 12085] <... pread resumed> "\360\233:\315\267\177\0\0\31\0\0\0\32\0\0\0 \336\200\1\0\0\0\0\230\324\203\1\0\0\0\0"..., 17800, 3301780) = 17800
[pid 12083] pread(7,  <unfinished ...>
[pid 12083] <... pread resumed> "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 18028, 3264523) = 18028
[pid 12083] pread(7,  <unfinished ...>
[pid 12083] <... pread resumed> "\0\0\0\0\0\0\0\0\0\0\0\0\250\350\203\1\0\0\0\0a\244\3669\267\177\0\0008\350\203\1"..., 19229, 3282551) = 19229
[pid 12085] pread(7,  <unfinished ...>
[pid 12085] <... pread resumed> "\0\0\0\0\2\0\0\0\0\0\0\0\377\377\377\377\377\377\377\377\0\0\0\0\0\0\0\0\0\0\0\0"..., 21116, 3223673) = 21116
[pid 12084] pread(7, "\377\377\377\377\0\0\0\0\0\0\0\0\0\0\0\0\1\0\1\1\1\0\0\0\260\221:\315\267\177\0\0"..., 21246, 3202427) = 21246
[pid 12081] write(1, "WTF? 18028:18028\n", 17WTF? 18028:18028
[pid 12081] write(1, "WTF? 19229:19229\n", 17WTF? 19229:19229

setTimeout(reader, 1000) instead of just reader provide the same results.