Fix tree hashing with nested empty directories (#1522)

When testing for directories to exclude from hashing, we must exclude
not only empty directories, but also directories that themselves contain
nothing but empty directories; in essence, we suppress adding
directories that have no files contained within their entire subtree.

While fixing this, it seemed prudent to eliminate the `names` argument
to `tree_hash()`, especially as it was not actually used to iterate
over.

Fixes JuliaLang/julia#33979 (comment)

src/GitTools.jl

@@ -250,17 +250,30 @@ function blob_hash(path::AbstractString, HashType = SHA.SHA1_CTX)
     return SHA.digest!(ctx)
 end
 
+"""
+    contains_no_files(root::AbstractString)
+
+Helper function to determine whether a directory contains no files; e.g. it is
+either empty or contains only other directories that contain nothing but other
+directories.  This is used to exclude directories from tree hashing.
+"""
+function contains_no_files(root::AbstractString)
+    for (root, dirs, files) in walkdir(root)
+        if !isempty(files)
+            return false
+        end
+    end
+    return true
+end
+
+
 """
     tree_hash(root::AbstractString)
 
 Calculate the git tree hash of a given path.  Note that attempting to take the
 tree hash of an empty directory will throw an error.
 """
-function tree_hash(
-    root::AbstractString,
-    names::Vector{String} = readdir(root);
-    HashType = SHA.SHA1_CTX,
-)
+function tree_hash(root::AbstractString; HashType = SHA.SHA1_CTX)
     entries = Tuple{String, Vector{UInt8}, GitMode}[]
     for f in readdir(root)
         # Skip `.git` directories
@@ -271,9 +284,11 @@ function tree_hash(
         filepath = abspath(root, f)
         mode = gitmode(filepath)
         if mode == mode_dir
-            names = readdir(filepath)
-            isempty(names) && continue
-            hash = tree_hash(filepath, names)
+            # If this directory contains no files, then skip it
+            contains_no_files(filepath) && continue
+
+            # Otherwise, hash it up!
+            hash = tree_hash(filepath)
         else
             hash = blob_hash(filepath)
         end

test/artifacts.jl

@@ -126,8 +126,11 @@ end
             end
 
             # Empty directories do nothing to effect the hash, so we create one with a
-            # random name to prove that it does not get hashed into the rest.
-            mkpath(joinpath(path, Random.randstring(8)))
+            # random name to prove that it does not get hashed into the rest.  Also, it
+            # turns out that life is cxomplex enough that we need to test the nested
+            # empty directories case as well.
+            rand_dir = joinpath(path, Random.randstring(8), "inner")
+            mkpath(rand_dir)
 
             # Symlinks are not followed, even if they point to directories
             symlink("foo3", joinpath(path, "foo3_link"))
@@ -148,7 +151,7 @@ end
                 cd(path) do
                     read(`git init .`)
                     read(`git add . `)
-                    read(`git commit -m "foo"`)
+                    read(`git commit --allow-empty -m "foo"`)
                     hash = chomp(String(read(`git log -1 --pretty='%T' HEAD`)))
                     println(hash)
                 end