[REPL] Fix shell completion error on unreadable symlink (#51851)

In macOS there is a file, `/usr/sbin/weakpass_edit` that is a symlink to
a directory that is only readable by `root`. Our REPL shell completion
attempts to call `isfile()` when entering the shell mode and pressing
`w`, throwing an ugly error into the REPL.

This PR fixes the root cause (skipping files that fail the `isfile()`
condition) and adds a test that synthesizes an analogous condition.

Author: staticfloat

stdlib/REPL/src/REPLCompletions.jl

@@ -329,8 +329,19 @@ function complete_path(path::AbstractString, pos::Int;
             for file in filesinpath
                 # In a perfect world, we would filter on whether the file is executable
                 # here, or even on whether the current user can execute the file in question.
-                if startswith(file, prefix) && isfile(joinpath(pathdir, file))
-                    push!(matches, file)
+                try
+                    if startswith(file, prefix) && isfile(joinpath(pathdir, file))
+                        push!(matches, file)
+                    end
+                catch e
+                    # `isfile()` can throw in rare cases such as when probing a
+                    # symlink that points to a file within a directory we do not
+                    # have read access to.
+                    if isa(e, Base.IOError)
+                        continue
+                    else
+                        rethrow()
+                    end
                 end
             end
         end

stdlib/REPL/test/replcompletions.jl

@@ -1119,28 +1119,35 @@ let s, c, r
     end
 
     # Tests detecting of files in the env path (in shell mode)
-    let path, file
-        path = tempdir()
-        unreadable = joinpath(tempdir(), "replcompletion-unreadable")
+    mktempdir() do path
+        unreadable = joinpath(path, "replcompletion-unreadable")
+        file = joinpath(path, "tmp-executable")
+        touch(file)
+        chmod(file, 0o755)
+        mkdir(unreadable)
+        hidden_file = joinpath(unreadable, "hidden")
+        touch(hidden_file)
 
-        try
-            file = joinpath(path, "tmp-executable")
-            touch(file)
-            chmod(file, 0o755)
-            mkdir(unreadable)
-            chmod(unreadable, 0o000)
+        # Create symlink to a file that is in an unreadable directory
+        chmod(hidden_file, 0o755)
+        chmod(unreadable, 0o000)
+        symlink(hidden_file, joinpath(path, "replcompletions-link"))
 
+        try
             # PATH can also contain folders which we aren't actually allowed to read.
             withenv("PATH" => string(path, ":", unreadable)) do
                 s = "tmp-execu"
                 c,r = test_scomplete(s)
                 @test "tmp-executable" in c
                 @test r == 1:9
                 @test s[r] == "tmp-execu"
+
+                c,r = test_scomplete("replcompletions-link")
+                @test isempty(c)
             end
         finally
-            rm(file)
-            rm(unreadable)
+            # If we don't fix the permissions here, our cleanup fails.
+            chmod(unreadable, 0o700)
         end
     end