When setting JULIA_DEPOT_PATH to /path:, omit the default user de…

…pot (#51448)

This commit slightly changes how the `DEPOT_PATH` works when overriden using
the env var `JULIA_DEPOT_PATH`, now omitting the default user depot when
specifying a path.

Before:

```
❯ JULIA_DEPOT_PATH=/foo: \
  julia-1.10 -e 'display(DEPOT_PATH)'
4-element Vector{String}:
 "/foo"
 "/home/tim/.julia"
 "/path/to/julia/local/share/julia"
 "/path/to/julia/share/julia"
```

After:

```
❯ JULIA_DEPOT_PATH=/foo: \
  julia-pr -e 'display(DEPOT_PATH)'
3-element Vector{String}:
 "/foo"
 "/path/to/julia/local/share/julia"
 "/path/to/julia/share/julia"
```

This makes it possibly to create a new isolated depot by simply
setting `JULIA_DEPOT_PATH` to `/path:`, while still being able
to load resources that are bundled with Julia.

For a full discussion, see the PR.

NEWS.md

@@ -34,6 +34,9 @@ Language changes
  pkgimage caches for all other packages than the package being tested, likely meaning faster test
  execution. ([#52123])
 
+* Specifying a path in `JULIA_DEPOT_PATH` now results in the expansion of empty strings to
+ omit the default user depot ([#51448]).
+
 Compiler/Runtime improvements
 -----------------------------
 * Updated GC heuristics to count allocated pages instead of individual objects ([#50144]).

base/initdefs.jl

@@ -94,9 +94,7 @@ 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")
@@ -108,17 +106,31 @@ 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

doc/src/manual/environment-variables.md

@@ -136,23 +136,30 @@ as Julia's code loading mechanisms, look for package registries, installed
 packages, named environments, repo clones, cached compiled package images,
 configuration files, and the default location of the REPL's history file.
 
-Unlike the shell `PATH` variable but similar to [`JULIA_LOAD_PATH`](@ref JULIA_LOAD_PATH), empty entries in
-[`JULIA_DEPOT_PATH`](@ref JULIA_DEPOT_PATH) are expanded to the default value of `DEPOT_PATH`. This allows
-easy appending, prepending, etc. of the depot path value in shell scripts regardless
-of whether [`JULIA_DEPOT_PATH`](@ref JULIA_DEPOT_PATH) is already set or not. For example, to prepend the
-directory `/foo/bar` to `DEPOT_PATH` just do
+Unlike the shell `PATH` variable but similar to [`JULIA_LOAD_PATH`](@ref JULIA_LOAD_PATH),
+empty entries in [`JULIA_DEPOT_PATH`](@ref JULIA_DEPOT_PATH) are expanded to the default
+value of `DEPOT_PATH`, excluding the user depot. This allows easy overriding of the user
+depot, while still retaining access to resources that are bundled with Julia, like cache
+files, artifacts, etc. For example, to switch the user depot to `/foo/bar` just do
 ```sh
-export JULIA_DEPOT_PATH="/foo/bar:$JULIA_DEPOT_PATH"
+export JULIA_DEPOT_PATH="/foo/bar:"
 ```
-If the [`JULIA_DEPOT_PATH`](@ref JULIA_DEPOT_PATH) environment variable is already set, its old value will be
-prepended with `/foo/bar`. On the other hand, if [`JULIA_DEPOT_PATH`](@ref JULIA_DEPOT_PATH) is not set, then
-it will be set to `/foo/bar:` which will have the effect of prepending `/foo/bar` to
-the default depot path. If [`JULIA_DEPOT_PATH`](@ref JULIA_DEPOT_PATH) is set to the empty string, it expands
-to an empty `DEPOT_PATH` array. In other words, the empty string is interpreted as a
-zero-element array, not a one-element array of the empty string. This behavior was
-chosen so that it would be possible to set an empty depot path via the environment
-variable. If you want the default depot path, either unset the environment variable
-or if it must have a value, set it to the string `:`.
+All package operations, like cloning registrise or installing packages, will now write to
+`/foo/bar`, but since the empty entry is expanded to the default system depot, any bundled
+resources will still be available. If you really only want to use the depot at `/foo/bar`,
+and not load any bundled resources, simply set the environment variable to `/foo/bar`
+without the trailing colon.
+
+There are two exceptions to the above rule. First, if [`JULIA_DEPOT_PATH`](@ref
+JULIA_DEPOT_PATH) is set to the empty string, it expands to an empty `DEPOT_PATH` array. In
+other words, the empty string is interpreted as a zero-element array, not a one-element
+array of the empty string. This behavior was chosen so that it would be possible to set an
+empty depot path via the environment variable.
+
+Second, if no user depot is specified in [`JULIA_DEPOT_PATH`](@ref JULIA_DEPOT_PATH), then
+the empty entry is expanded to the default depot *including* the user depot. This makes
+it possible to use the default depot, as if the environment variable was unset, by setting
+it to the string `:`.
 
 !!! note
 

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)