path/filepath: detect Windows CONIN$ and CONOUT$ paths in IsLocal

CreateFile creates a handle to the console input or screen buffer
when opening a file named CONIN$ or CONOUT$:

https://learn.microsoft.com/en-us/windows/win32/api/fileapi/nf-fileapi-createfilea#consoles

Detect these paths as non-local.

For #56219.

Change-Id: Ib09e76a110d6ec09aef8038074b9bcbae09d00d7
Reviewed-on: https://go-review.googlesource.com/c/go/+/451657
Run-TryBot: Damien Neil <dneil@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>

src/path/filepath/path_test.go

@@ -174,6 +174,11 @@ var winislocaltests = []IsLocalTest{
 	{`C:`, false},
 	{`C:\a`, false},
 	{`..\a`, false},
+	{`CONIN$`, false},
+	{`conin$`, false},
+	{`CONOUT$`, false},
+	{`conout$`, false},
+	{`dollar$`, true}, // not a special file name
 }
 
 var plan9islocaltests = []IsLocalTest{

src/path/filepath/path_windows.go

@@ -20,7 +20,7 @@ func toUpper(c byte) byte {
 	return c
 }
 
-// isReservedName reports if name is a Windows reserved device name.
+// isReservedName reports if name is a Windows reserved device name or a console handle.
 // It does not detect names with an extension, which are also reserved on some Windows versions.
 //
 // For details, search for PRN in
@@ -34,6 +34,17 @@ func isReservedName(name string) bool {
 			return len(name) == 4 && '1' <= name[3] && name[3] <= '9'
 		}
 	}
+	// Passing CONIN$ or CONOUT$ to CreateFile opens a console handle.
+	// https://learn.microsoft.com/en-us/windows/win32/api/fileapi/nf-fileapi-createfilea#consoles
+	//
+	// While CONIN$ and CONOUT$ aren't documented as being files,
+	// they behave the same as CON. For example, ./CONIN$ also opens the console input.
+	if len(name) == 6 && name[5] == '$' && strings.EqualFold(name, "CONIN$") {
+		return true
+	}
+	if len(name) == 7 && name[6] == '$' && strings.EqualFold(name, "CONOUT$") {
+		return true
+	}
 	return false
 }