Resolve paths before comparing input/output destination (#2878)

This fixes an issue where if you try to write to the same destination as the
input file but you are not using absolute (or the same relative path) for both
the input and output, sharp/vips might produce errors such as:

someFile.jpg: unable to open for write
unix error: No such file or directory

lib/output.js

@@ -1,5 +1,6 @@
 'use strict';
 
+const path = require('path');
 const is = require('./is');
 const sharp = require('./sharp');
 
@@ -59,7 +60,7 @@ function toFile (fileOut, callback) {
   let err;
   if (!is.string(fileOut)) {
     err = new Error('Missing output file path');
-  } else if (this.options.input.file === fileOut) {
+  } else if (is.string(this.options.input.file) && path.resolve(this.options.input.file) === path.resolve(fileOut)) {
     err = new Error('Cannot use same file for input and output');
   } else if (this.options.formatOut === 'input' && fileOut.toLowerCase().endsWith('.gif') && !this.constructor.format.magick.output.file) {
     err = errMagickSave;

test/unit/io.js

@@ -1,6 +1,7 @@
 'use strict';
 
 const fs = require('fs');
+const path = require('path');
 const assert = require('assert');
 const rimraf = require('rimraf');
 
@@ -316,6 +317,48 @@ describe('Input/output', function () {
     });
   });
 
+  it('Fail when output File is input File (relative output, absolute input)', function (done) {
+    const relativePath = path.relative(process.cwd(), fixtures.inputJpg);
+    sharp(fixtures.inputJpg).toFile(relativePath, function (err) {
+      assert(err instanceof Error);
+      assert.strictEqual('Cannot use same file for input and output', err.message);
+      done();
+    });
+  });
+
+  it('Fail when output File is input File via Promise (relative output, absolute input)', function (done) {
+    const relativePath = path.relative(process.cwd(), fixtures.inputJpg);
+    sharp(fixtures.inputJpg).toFile(relativePath).then(function (data) {
+      assert(false);
+      done();
+    }).catch(function (err) {
+      assert(err instanceof Error);
+      assert.strictEqual('Cannot use same file for input and output', err.message);
+      done();
+    });
+  });
+
+  it('Fail when output File is input File (relative input, absolute output)', function (done) {
+    const relativePath = path.relative(process.cwd(), fixtures.inputJpg);
+    sharp(relativePath).toFile(fixtures.inputJpg, function (err) {
+      assert(err instanceof Error);
+      assert.strictEqual('Cannot use same file for input and output', err.message);
+      done();
+    });
+  });
+
+  it('Fail when output File is input File via Promise (relative input, absolute output)', function (done) {
+    const relativePath = path.relative(process.cwd(), fixtures.inputJpg);
+    sharp(relativePath).toFile(fixtures.inputJpg).then(function (data) {
+      assert(false);
+      done();
+    }).catch(function (err) {
+      assert(err instanceof Error);
+      assert.strictEqual('Cannot use same file for input and output', err.message);
+      done();
+    });
+  });
+
   it('Fail when output File is empty', function (done) {
     sharp(fixtures.inputJpg).toFile('', function (err) {
       assert(err instanceof Error);