Fix temp-path related failures.

- The fix in bd36730 (which addresses Windows Update changes that
  cause the root disk to run out of space in some deployment scenarios)
  failed for some deployments (with a "cannot find path" error
  originating from groot). We suspect (but are having trouble
  validating) that the problem is incomplete percolation of the
  SystemTemp env var.
- This commit reverts bd36730 in favor of sym-linking
  C:\Windows\SystemTemp, another way of achieving the desired outcome:
  making sure lifecycle scripts write temp data to the ephermeral disk.
  A deployment configuration that failed with the changes in bd36730
  completes successfully with this change.
- A possible risk here is anything that might have previously
  _explicitly_ written to C:\Windows\SystemTemp will now appear instead
  in C:\var\vcap\data\tmp, which may have different permissions -- an
  incremental change to bd36730, which would have seen processes
  previously acquiring a temp dir from system calls writing to
  C:\var\vcap\data\tmp. Given that we would expect processes prior to
  the Windows Update change to _also_ have been writing to
  C:\var\vcap\data\tmp (via the TEMP and TMP env vars), this would not
  seem to overall change current behavior, although could be surprising
  with regards to future expectations around temp dirs.
- n.b.: our test does not check that existing files in SystemTemp are
  removed, but we would expect this behavior to be the case (because the
  file system fake currently does NOT emulate this behavior, although
  this behavior IS expected from the concrete file system). At the
  moment modifying the fake to include this behavior doesn't seem to
  provide much benefit as this makes the test is more about fake
  behavior than concrete behavior.

Author: ragaskar

platform/windows_platform.go

@@ -582,6 +582,12 @@ func (p WindowsPlatform) SetupTmpDir() error {
 		return bosherr.WrapError(err, "Creating temp dir")
 	}
 
+	systemTemp := os.TempDir()
+	err = p.fs.Symlink(boshTmpDir, systemTemp)
+	if err != nil {
+		return bosherr.WrapError(err, fmt.Sprintf("Creating symlink from %s to %s", systemTemp, boshTmpDir))
+	}
+
 	err = os.Setenv("TMP", boshTmpDir)
 	if err != nil {
 		return bosherr.WrapError(err, "Setting TMP")
@@ -592,11 +598,6 @@ func (p WindowsPlatform) SetupTmpDir() error {
 		return bosherr.WrapError(err, "Setting TEMP")
 	}
 
-	err = os.Setenv("SystemTemp", boshTmpDir)
-	if err != nil {
-		return bosherr.WrapError(err, "Setting SystemTemp")
-	}
-
 	return nil
 }
 

platform/windows_platform_test.go

@@ -172,9 +172,14 @@ var _ = Describe("WindowsPlatform", func() {
 
 	Describe("SetupTmpDir", func() {
 		var (
-			OrigTMP  = os.Getenv("TMP")
-			OrigTEMP = os.Getenv("TEMP")
+			OrigTMP    = os.Getenv("TMP")
+			OrigTEMP   = os.Getenv("TEMP")
+			systemTemp = os.TempDir()
 		)
+		BeforeEach(func() {
+			err := fs.MkdirAll(systemTemp, 0755)
+			Expect(err).NotTo(HaveOccurred())
+		})
 		AfterEach(func() {
 			os.Setenv("TMP", OrigTMP)   //nolint:errcheck
 			os.Setenv("TEMP", OrigTEMP) //nolint:errcheck
@@ -197,14 +202,28 @@ var _ = Describe("WindowsPlatform", func() {
 			Expect(err.Error()).To(ContainSubstring("fake-mkdir-error"))
 		})
 
-		It("sets TMP, TEMP, and SystemTemp environment variables so that children of this process will use new temp dir", func() {
+		It("sets TMP, TEMP environment variables so that children of this process will use new temp dir", func() {
 			err := platform.SetupTmpDir()
 			Expect(err).NotTo(HaveOccurred())
 
 			fakeTmpDir := filepath.FromSlash("/fake-dir/data/tmp")
 			Expect(os.Getenv("TMP")).To(Equal(fakeTmpDir))
 			Expect(os.Getenv("TEMP")).To(Equal(fakeTmpDir))
-			Expect(os.Getenv("SystemTemp")).To(Equal(fakeTmpDir))
+		})
+
+		It("symlinks os.TempDir to $base-dir/data/tmp", func() {
+			fileStats := fs.GetFileTestStat(systemTemp)
+			Expect(fileStats).NotTo(BeNil())
+			Expect(fileStats.FileType).To(Equal(fakesys.FakeFileTypeDir))
+
+			err := platform.SetupTmpDir()
+			Expect(err).NotTo(HaveOccurred())
+
+			fileStats = fs.GetFileTestStat(systemTemp)
+			Expect(fileStats).NotTo(BeNil())
+			Expect(fileStats.FileType).To(Equal(fakesys.FakeFileTypeSymlink))
+
+			Expect(fileStats.SymlinkTarget).To(Equal("/fake-dir/data/tmp"))
 		})
 
 		It("returns error if setting TMPDIR errs", func() {