Fix which behavior when passed an empty string (#33150)

* Fix behavior of Sys.which when passed an empty String argument

* Added test to check for fixed Sys.which behavior with empty string input

* Added test to check that Sys.which returns nothing when passed a blank
string

* Ensure that Sys.which returns a regular file and never a directory

* Moved new Sys.which tests into test/spawn.jl alongside the existing ones

* Remove new which tests from test/sysinfo.jl (they've moved to
test/spawn.jl)

base/sysinfo.jl

@@ -459,6 +459,9 @@ for executable permissions only (with `.exe` and `.com` extensions added on
 Windows platforms); no searching of `PATH` is performed.
 """
 function which(program_name::String)
+    if isempty(program_name)
+       return nothing
+    end
     # Build a list of program names that we're going to try
     program_names = String[]
     base_pname = basename(program_name)
@@ -501,7 +504,7 @@ function which(program_name::String)
         for pname in program_names
             program_path = joinpath(path_dir, pname)
             # If we find something that matches our name and we can execute
-            if isexecutable(program_path)
+            if isfile(program_path) && isexecutable(program_path)
                 return realpath(program_path)
             end
         end

test/spawn.jl

@@ -556,6 +556,10 @@ withenv("PATH" => "$(Sys.BINDIR)$(psep)$(ENV["PATH"])") do
     @test Sys.which(julia_exe) == realpath(julia_exe)
 end
 
+# Check that which behaves correctly when passed an empty string
+@test isnothing(Base.Sys.which(""))
+
+
 mktempdir() do dir
     withenv("PATH" => "$(dir)$(psep)$(ENV["PATH"])") do
         # Test that files lacking executable permissions fail Sys.which
@@ -572,8 +576,15 @@ mktempdir() do dir
             @test Sys.which(foo_path) === nothing
         end
 
+    end
+
+    # Ensure these tests are done only with a PATH of known contents
+    withenv("PATH" => "$(dir)") do
         # Test that completely missing files also return nothing
         @test Sys.which("this_is_not_a_command") === nothing
+
+        # Check that which behaves correctly when passed a blank string
+        @test isnothing(Base.Sys.which(" "))
     end
 end