Introduce fs.FileExists

[andymoe]

Summary

Adds a the method FileExists to the fs sub package. This is code we found ourselves needing more than a couple places in the conda-env-update buildpack.

Also alphabetized suite call order in fs/init_test.go

Use Case

lockfilePresent, err := fs.FileExists("/path/to/lockfile.txt") 
if err != nil {
  panic("Et tu, lockfile?")
}

if lockfilePresent {
  fancypackage.DoFancyThingsWithLockfile()
}

Note: this PR does not have test cases for using this method against a directory. While we have more test coverage than libbuildpack does I've not been able to figure out how to correctly test this case properly. The additional (broken/wrong) tests for a fs.FileExists(/my/missing/directory) are on this branch if you have ideas/feedback.

[ryanmoran]

I'd like to see a couple changes:

1. Can we change the method to fs.Exists since it should apply to more than just files?
2. I've figured out the issues with the directory tests. Mostly it was caused by the definition of subDir in the wrong scope. Here is a diff of the changes I made:

                context("when we are testing for a directory", func() {
-                       var subDir = filepath.Join(dirPath, "sub-dir")
+                       var subDir string
+
+                       it.Before(func() {
+                               subDir = filepath.Join(dirPath, "sub-dir")
+                               Expect(os.Mkdir(subDir, os.ModePerm)).To(Succeed())
+                       })
+
                        context("and a directory exists", func() {
                                it("returns true", func() {
-                                       Expect(os.Mkdir(subDir, os.ModePerm)).To(Succeed())
-                                       Expect(fs.FileExists(dirPath)).To(BeTrue())
+                                       Expect(fs.FileExists(subDir)).To(BeTrue())
                                })
                        })
+
                        context("when the directory does not exist", func() {
                                it.Before(func() {
-                                       Expect(os.RemoveAll(dirPath)).To(Succeed())
+                                       Expect(os.RemoveAll(subDir)).To(Succeed())
                                })

                                it("returns false", func() {
-                                       Expect(fs.FileExists(dirPath)).To(BeFalse())
+                                       Expect(fs.FileExists(subDir)).To(BeFalse())
                                })
                        })

@@ -97,12 +104,10 @@ func testFileExists(t *testing.T, context spec.G, it spec.S) {
                                        Expect(os.Chmod(dirPath, os.ModePerm)).To(Succeed())
                                })

-                               // With this setup we keep getting nil error which I don't understand.
-                               // I'm clearly missing something.
                                it("returns false and an error", func() {
-                                       exits, err := fs.FileExists(subDir)
-                                       Expect(err.Error()).To(ContainSubstring("<syscall.Errno>0xd"))
-                                       Expect(exits).To(BeFalse())
+                                       exists, err := fs.FileExists(subDir)
+                                       Expect(err).To(MatchError(ContainSubstring("permission denied")))
+                                       Expect(exists).To(BeFalse())
                                })
                        })
                })

[andymoe]

Thanks! I'll get this fixed up. I really got myself confused with the scoping there...

[ForestEckhardt]

@andymoe @ryanmoran Is the only thing that this is waiting on changing the name of the function to be more generic? I would love to move this along because the ask seems relatively straight forward.

[andymoe]

Yeah, sorry. I had some issues with my tests etc too and I never got back to it and sorted it out locality.

…

On Wed, Jun 9, 2021 at 12:01 PM Forest Eckhardt ***@***.***> wrote: @andymoe <https://github.com/andymoe> @ryanmoran <https://github.com/ryanmoran> Is the only thing that this is waiting on changing the name of the function to be more generic? I would love to move this along because the ask seems relatively straight forward. — You are receiving this because you were mentioned. Reply to this email directly, view it on GitHub <#160 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AAACTA4IJWRJVH7QL6HHLGDTR63APANCNFSM43CTH4RA> .

[ForestEckhardt]

@andymoe Bump

[sophiewigmore]

Drafting this for now since there has not been a code update since April.