Enable watching directories

Fixes #470

src/Revise.jl

@@ -143,9 +143,11 @@ You can use the return value `key` to remove the callback later
 to `Revise.add_callback` with `key=key`.
 """
 function add_callback(f, files, modules=nothing; key=gensym())
+    fix_trailing(path) = isdir(path) ? joinpath(path, "") : path   # insert a trailing '/' if missing, see https://github.com/timholy/Revise.jl/issues/470#issuecomment-633298553
+
     remove_callback(key)
 
-    files = map(abspath, files)
+    files = map(fix_trailing, map(abspath, files))
     init_watching(files)
 
     if modules !== nothing
@@ -928,7 +930,7 @@ includet(file::AbstractString) = includet(Main, file)
     entr(f, files; postpone=false, pause=0.02)
     entr(f, files, modules; postpone=false, pause=0.02)
 
-Execute `f()` whenever files listed in `files`, or code in `modules`, updates.
+Execute `f()` whenever files or directories listed in `files`, or code in `modules`, updates.
 `entr` will process updates (and block your command line) until you press Ctrl-C.
 Unless `postpone` is `true`, `f()` will be executed also when calling `entr`,
 regardless of file changes. The `pause` is the period (in seconds) that `entr`

test/runtests.jl

@@ -2949,6 +2949,35 @@ do_test("entr") && @testset "entr" begin
         @test isa(err, LoadError)
         @test err.error.msg == "stop"
     end
+
+    # Watch directories (#470)
+    dn = joinpath(tempdir(), randtmp())
+    mkdir(dn)
+    fn = joinpath(dn, "trigger.txt")
+    open(fn, "w") do io
+        println(io, "blank")
+    end
+    counter = Ref(0)
+    stop = Ref(false)
+    try
+        @sync begin
+            @async begin
+                entr([dn]) do
+                    counter[] += 1
+                    stop[] && error("stopping")
+                end
+            end
+            sleep(mtimedelay)
+            touch(fn)
+            sleep(mtimedelay)
+            @test counter[] == 1
+            stop[] = true
+            touch(fn)
+            sleep(mtimedelay)
+        end
+    catch
+        @test counter[] == 2
+    end
 end
 
 const A354_result = Ref(0)