Use libuv for tempdir function (#31434)

Fixes the Windows tempdir function returning a path with the trailing slash.

Author: musm

base/file.jl

@@ -424,18 +424,34 @@ function touch(path::AbstractString)
     path
 end
 
-const temp_prefix = "jl_"
-
-if Sys.iswindows()
+"""
+    tempdir()
 
+Gets the path of the temporary directory. On Windows, `tempdir()` uses the first environment
+variable found in the ordered list `TMP`, `TEMP`, `USERPROFILE`. On all other operating
+systems, `tempdir()` uses the first environment variable found in the ordered list `TMPDIR`,
+`TMP`, `TEMP`, and `TEMPDIR`. If none of these are found, the path `"/tmp"` is used.
+"""
 function tempdir()
-    temppath = Vector{UInt16}(undef, 32767)
-    lentemppath = ccall(:GetTempPathW, stdcall, UInt32, (UInt32, Ptr{UInt16}), length(temppath), temppath)
-    windowserror("GetTempPath", lentemppath >= length(temppath) || lentemppath == 0)
-    resize!(temppath, lentemppath)
-    return transcode(String, temppath)
+    buf = Base.StringVector(AVG_PATH - 1) # space for null-terminator implied by StringVector
+    sz = RefValue{Csize_t}(length(buf) + 1) # total buffer size including null
+    while true
+        rc = ccall(:uv_os_tmpdir, Cint, (Ptr{UInt8}, Ptr{Csize_t}), buf, sz)
+        if rc == 0
+            resize!(buf, sz[])
+            return String(buf)
+        elseif rc == Base.UV_ENOBUFS
+            resize!(buf, sz[] - 1)  # space for null-terminator implied by StringVector
+        else
+            uv_error(:tmpdir, rc)
+        end
+    end
 end
 
+const temp_prefix = "jl_"
+
+if Sys.iswindows()
+
 function _win_tempname(temppath::AbstractString, uunique::UInt32)
     tempp = cwstring(temppath)
     temppfx = cwstring(temp_prefix)
@@ -481,9 +497,6 @@ function tempname()
     return s
 end
 
-# Obtain a temporary directory's path.
-tempdir() = dirname(tempname())
-
 # Create and return the name of a temporary file along with an IOStream
 function mktemp(parent=tempdir())
     b = joinpath(parent, temp_prefix * "XXXXXX")
@@ -496,13 +509,6 @@ end
 end # os-test
 
 
-"""
-    tempdir()
-
-Obtain the path of a temporary directory (possibly shared with other processes).
-"""
-tempdir()
-
 """
     tempname()
 

test/file.jl

@@ -244,8 +244,39 @@ close(s)
     end
 end
 
-my_tempdir = tempdir()
-@test isdir(my_tempdir) == true
+@testset "tempdir" begin
+    my_tempdir = tempdir()
+    @test isdir(my_tempdir)
+    @test my_tempdir[end] != '/'
+    @test my_tempdir[end] != '\\'
+
+    var =  Sys.iswindows() ? "TMP" : "TMPDIR"
+    PATH_PREFIX = Sys.iswindows() ? "C:\\" : "/tmp/"
+    # Warning: On Windows uv_os_tmpdir internally calls GetTempPathW. The max string length for
+    # GetTempPathW is 261 (including the implied trailing backslash), not the typical length 259.
+    # We thus use 260 (with implied trailing slash backlash this then gives 261 chars) and
+    # subtract 9 to account for i = 0:9.
+    MAX_PATH = (Sys.iswindows() ? 260-9 : 1024) - length(PATH_PREFIX)
+    for i = 0:8
+        local tmp = PATH_PREFIX * "x"^MAX_PATH * "123456789"[1:i]
+        @test withenv(var => tmp) do
+            tempdir()
+        end == (tmp)
+    end
+    for i = 9
+        local tmp = PATH_PREFIX * "x"^MAX_PATH * "123456789"[1:i]
+        if Sys.iswindows()
+            # libuv bug
+            @test_broken withenv(var => tmp) do
+                tempdir()
+            end == tmp
+        else
+            @test withenv(var => tmp) do
+                tempdir()
+            end == tmp
+        end
+    end
+end
 
 let path = tempname()
     # issue #9053