Win: Add GetHostNameW fallback for win7 using gethostname

Author: Fulgen301

library/std/src/sys/pal/windows/c.rs

@@ -227,6 +227,18 @@ compat_fn_with_fallback! {
     }
 }
 
+#[cfg(target_vendor = "win7")]
+compat_fn_with_fallback! {
+    pub static WS2_32: &CStr = c"ws2_32";
+
+    #[cfg(target_vendor = "win7")]
+    pub fn GetHostNameW(name: PWSTR, namelen: i32) -> i32 {
+        unsafe {
+            crate::sys::winsock::hostname_fallback(name, namelen)
+        }
+    }
+}
+
 cfg_select! {
     target_vendor = "uwp" => {
         windows_targets::link_raw_dylib!("ntdll.dll" "system" fn NtCreateFile(filehandle : *mut HANDLE, desiredaccess : FILE_ACCESS_RIGHTS, objectattributes : *const OBJECT_ATTRIBUTES, iostatusblock : *mut IO_STATUS_BLOCK, allocationsize : *const i64, fileattributes : FILE_FLAGS_AND_ATTRIBUTES, shareaccess : FILE_SHARE_MODE, createdisposition : NTCREATEFILE_CREATE_DISPOSITION, createoptions : NTCREATEFILE_CREATE_OPTIONS, eabuffer : *const core::ffi::c_void, ealength : u32) -> NTSTATUS);

library/std/src/sys/pal/windows/c/bindings.txt

@@ -33,6 +33,7 @@ CONSOLE_MODE
 CONSOLE_READCONSOLE_CONTROL
 CONTEXT
 CopyFileExW
+CP_ACP
 CP_UTF8
 CREATE_ALWAYS
 CREATE_BREAKAWAY_FROM_JOB
@@ -2170,6 +2171,7 @@ GetFileType
 GETFINALPATHNAMEBYHANDLE_FLAGS
 GetFinalPathNameByHandleW
 GetFullPathNameW
+gethostname
 GetHostNameW
 GetLastError
 GetModuleFileNameW

library/std/src/sys/pal/windows/c/windows_sys.rs

@@ -130,6 +130,7 @@ windows_targets::link!("ws2_32.dll" "system" fn closesocket(s : SOCKET) -> i32);
 windows_targets::link!("ws2_32.dll" "system" fn connect(s : SOCKET, name : *const SOCKADDR, namelen : i32) -> i32);
 windows_targets::link!("ws2_32.dll" "system" fn freeaddrinfo(paddrinfo : *const ADDRINFOA));
 windows_targets::link!("ws2_32.dll" "system" fn getaddrinfo(pnodename : PCSTR, pservicename : PCSTR, phints : *const ADDRINFOA, ppresult : *mut *mut ADDRINFOA) -> i32);
+windows_targets::link!("ws2_32.dll" "system" fn gethostname(name : PSTR, namelen : i32) -> i32);
 windows_targets::link!("ws2_32.dll" "system" fn getpeername(s : SOCKET, name : *mut SOCKADDR, namelen : *mut i32) -> i32);
 windows_targets::link!("ws2_32.dll" "system" fn getsockname(s : SOCKET, name : *mut SOCKADDR, namelen : *mut i32) -> i32);
 windows_targets::link!("ws2_32.dll" "system" fn getsockopt(s : SOCKET, level : i32, optname : i32, optval : PSTR, optlen : *mut i32) -> i32);
@@ -444,6 +445,7 @@ pub struct CONTEXT_0_0 {
 pub type CONTEXT_FLAGS = u32;
 pub type COPYFILE_FLAGS = u32;
 pub type COPYPROGRESSROUTINE_PROGRESS = u32;
+pub const CP_ACP: u32 = 0u32;
 pub const CP_UTF8: u32 = 65001u32;
 pub const CREATE_ALWAYS: FILE_CREATION_DISPOSITION = 2u32;
 pub const CREATE_BREAKAWAY_FROM_JOB: PROCESS_CREATION_FLAGS = 16777216u32;

library/std/src/sys/pal/windows/winsock.rs

@@ -78,3 +78,46 @@ where
 {
     cvt(f())
 }
+
+#[cfg(target_vendor = "win7")]
+pub unsafe fn hostname_fallback(name: c::PWSTR, namelen: i32) -> i32 {
+    assert!(namelen >= 1);
+
+    // The documentation of gethostname says that a buffer size of 256 is
+    // always enough.
+    let mut buffer = [const { mem::MaybeUninit::<u8>::uninit() }; 256];
+
+    // SAFETY: these parameters specify a valid, writable region of memory.
+    unsafe {
+        if c::gethostname(buffer.as_mut_ptr().cast(), buffer.len() as i32) == c::SOCKET_ERROR {
+            return c::SOCKET_ERROR;
+        }
+
+        // Subtract one to leave space for the null terminator, as MultiByteToWideChar doesn't terminate the output buffer
+        // if the number of output characters is equal to the buffer length.
+        let len = c::MultiByteToWideChar(
+            c::CP_ACP,
+            c::MB_ERR_INVALID_CHARS,
+            buffer.as_mut_ptr().cast(),
+            -1,
+            name,
+            namelen - 1,
+        );
+
+        if len <= 0 {
+            // GetHostNameW reports WSAEFAULT if the buffer is too small
+            if c::GetLastError() == c::ERROR_INSUFFICIENT_BUFFER {
+                c::SetLastError(c::WSAEFAULT as _);
+            }
+            return c::SOCKET_ERROR;
+        }
+
+        // Ensure the output is always null terminated.
+        // If MultiByteToWideChar has already written a null terminator, that null terminator will be included in len
+        // and this will add a second one, but writing a zero is cheap enough to omit the length comparison.
+        name.add(len as _).write(0);
+    }
+
+    // Success
+    0
+}