make Windows realpath consistent with Mac and Unix (#30611)

base/path.jl

@@ -333,17 +333,26 @@ current directory if necessary. Equivalent to `abspath(joinpath(path, paths...))
 abspath(a::AbstractString, b::AbstractString...) = abspath(joinpath(a,b...))
 
 if Sys.iswindows()
+
 function realpath(path::AbstractString)
-    p = cwstring(path)
-    buf = zeros(UInt16, length(p))
-    while true
-        n = ccall((:GetFullPathNameW, "kernel32"), stdcall,
-            UInt32, (Ptr{UInt16}, UInt32, Ptr{UInt16}, Ptr{Cvoid}),
-            p, length(buf), buf, C_NULL)
-        windowserror(:realpath, n == 0)
-        x = n < length(buf) # is the buffer big enough?
-        resize!(buf, n) # shrink if x, grow if !x
-        x && return transcode(String, buf)
+    h = ccall(:CreateFileW, stdcall, Int, (Cwstring, UInt32, UInt32, Ptr{Cvoid}, UInt32, UInt32, Int),
+                path, 0, 0x03, C_NULL, 3, 0x02000000, 0)
+    windowserror(:realpath, h == -1)
+    try
+        buf = Array{UInt16}(undef, 256)
+        oldlen = len = length(buf)
+        while len >= oldlen
+            len = ccall(:GetFinalPathNameByHandleW, stdcall, UInt32, (Int, Ptr{UInt16}, UInt32, UInt32),
+                            h, buf, (oldlen=len)-1, 0x0)
+            windowserror(:realpath, iszero(len))
+            resize!(buf, len) # strips NUL terminator on last call
+        end
+        if 4 < len < 264 && 0x005c == buf[1] == buf[2] == buf[4] && 0x003f == buf[3]
+            Base._deletebeg!(buf, 4) # omit \\?\ prefix for paths < MAXPATH in length
+        end
+        return transcode(String, buf)
+    finally
+        windowserror(:realpath, iszero(ccall(:CloseHandle, stdcall, Cint, (Int,), h)))
     end
 end
 
@@ -376,6 +385,10 @@ end # os-test
     realpath(path::AbstractString) -> AbstractString
 
 Canonicalize a path by expanding symbolic links and removing "." and ".." entries.
+On case-insensitive case-preserving filesystems (typically Mac and Windows), the
+filesystem's stored case for the path is returned.
+
+(This function throws an exception if `path` does not exist in the filesystem.)
 """
 realpath(path::AbstractString)
 

test/file.jl

@@ -1039,8 +1039,21 @@ mktempdir() do dir
     @test isdir(namepath)
 end
 
-
-
+# issue #30588
+@test realpath(".") == realpath(pwd())
+mktempdir() do dir
+    cd(dir) do
+        path = touch("FooBar.txt")
+        @test ispath(realpath(path))
+        if ispath(uppercase(path)) # case-insensitive filesystem
+            @test realpath(path) == realpath(uppercase(path)) == realpath(lowercase(path)) ==
+                  realpath(uppercase(realpath(path))) == realpath(lowercase(realpath(path)))
+            @test basename(realpath(uppercase(path))) == path
+        end
+        rm(path)
+        @test_throws SystemError realpath(path)
+    end
+end
 
 # issue #9687
 let n = tempname()