Change how bare separators are expanded in JULIA_DEPOT_PATH.

Instead of expanding to the default (homedir-based) + bundled (bindir-based) depot,
only use the bundled depot, so the env var can be used to select a different depot
while still being able to load bundled cache files.

base/initdefs.jl

@@ -86,9 +86,7 @@ See also [`JULIA_DEPOT_PATH`](@ref JULIA_DEPOT_PATH), and
 """
 const DEPOT_PATH = String[]
 
-function append_default_depot_path!(DEPOT_PATH)
-    path = joinpath(homedir(), ".julia")
-    path in DEPOT_PATH || push!(DEPOT_PATH, path)
+function append_bundled_depot_path!(DEPOT_PATH)
     path = abspath(Sys.BINDIR, "..", "local", "share", "julia")
     path in DEPOT_PATH || push!(DEPOT_PATH, path)
     path = abspath(Sys.BINDIR, "..", "share", "julia")
@@ -100,17 +98,31 @@ function init_depot_path()
     empty!(DEPOT_PATH)
     if haskey(ENV, "JULIA_DEPOT_PATH")
         str = ENV["JULIA_DEPOT_PATH"]
+
+        # explicitly setting JULIA_DEPOT_PATH to the empty string means using no depot
         isempty(str) && return
+
+        # otherwise, populate the depot path with the entries in JULIA_DEPOT_PATH,
+        # expanding empty strings to the bundled depot
+        populated = false
         for path in eachsplit(str, Sys.iswindows() ? ';' : ':')
             if isempty(path)
-                append_default_depot_path!(DEPOT_PATH)
+                append_bundled_depot_path!(DEPOT_PATH)
             else
                 path = expanduser(path)
                 path in DEPOT_PATH || push!(DEPOT_PATH, path)
+                populated = true
             end
         end
+
+        # backwards compatibility: if JULIA_DEPOT_PATH only contains empty entries
+        # (e.g., JULIA_DEPOT_PATH=':'), make sure to use the default depot
+        if !populated
+            pushfirst!(DEPOT_PATH, joinpath(homedir(), ".julia"))
+        end
     else
-        append_default_depot_path!(DEPOT_PATH)
+        push!(DEPOT_PATH, joinpath(homedir(), ".julia"))
+        append_bundled_depot_path!(DEPOT_PATH)
     end
     nothing
 end

test/loading.jl

@@ -721,16 +721,17 @@ end
 @testset "expansion of JULIA_DEPOT_PATH" begin
     s = Sys.iswindows() ? ';' : ':'
     tmp = "/this/does/not/exist"
-    DEFAULT = Base.append_default_depot_path!(String[])
+    default = joinpath(homedir(), ".julia")
+    bundled = Base.append_bundled_depot_path!(String[])
     cases = Dict{Any,Vector{String}}(
-        nothing => DEFAULT,
+        nothing => [default; bundled],
         "" => [],
-        "$s" => DEFAULT,
-        "$tmp$s" => [tmp; DEFAULT],
-        "$s$tmp" => [DEFAULT; tmp],
+        "$s" => [default; bundled],
+        "$tmp$s" => [tmp; bundled],
+        "$s$tmp" => [bundled; tmp],
         )
     for (env, result) in pairs(cases)
-        script = "DEPOT_PATH == $(repr(result)) || error()"
+        script = "DEPOT_PATH == $(repr(result)) || error(\"actual depot \" * join(DEPOT_PATH,':') * \" does not match expected depot \" * join($(repr(result)), ':'))"
         cmd = `$(Base.julia_cmd()) --startup-file=no -e $script`
         cmd = addenv(cmd, "JULIA_DEPOT_PATH" => env)
         cmd = pipeline(cmd; stdout, stderr)