libutil: Implement HANDLE-based lseek for Windows

For windows we should live fully in the HANDLE land instead
of converting back-n-forth (which sometimes is destructive).
Using native API is much better for this.

src/libutil-tests/file-system.cc

@@ -388,14 +388,13 @@ TEST(openFileEnsureBeneathNoSymlinks, works)
 TEST(createAnonymousTempFile, works)
 {
     auto fd = createAnonymousTempFile();
-    auto fd_ = fromDescriptorReadOnly(fd.get());
     writeFull(fd.get(), "test");
-    lseek(fd_, 0, SEEK_SET);
+    lseek(fd.get(), 0, SEEK_SET);
     FdSource source{fd.get()};
     EXPECT_EQ(source.drain(), "test");
-    lseek(fd_, 0, SEEK_END);
+    lseek(fd.get(), 0, SEEK_END);
     writeFull(fd.get(), "test");
-    lseek(fd_, 0, SEEK_SET);
+    lseek(fd.get(), 0, SEEK_SET);
     EXPECT_EQ(source.drain(), "testtest");
 }
 
@@ -406,9 +405,8 @@ TEST(createAnonymousTempFile, works)
 TEST(FdSource, restartWorks)
 {
     auto fd = createAnonymousTempFile();
-    auto fd_ = fromDescriptorReadOnly(fd.get());
     writeFull(fd.get(), "hello world");
-    lseek(fd_, 0, SEEK_SET);
+    lseek(fd.get(), 0, SEEK_SET);
     FdSource source{fd.get()};
     EXPECT_EQ(source.drain(), "hello world");
     source.restart();

src/libutil/include/nix/util/file-descriptor.hh

@@ -261,4 +261,14 @@ Descriptor openFileEnsureBeneathNoSymlinks(Descriptor dirFd, const CanonPath & p
 
 MakeError(EndOfFile, Error);
 
+#ifdef _WIN32
+
+/**
+ * Windows specific replacement for POSIX `lseek` that operates on a `HANDLE` and not
+ * a file descriptor.
+ */
+off_t lseek(Descriptor fd, off_t offset, int whence);
+
+#endif
+
 } // namespace nix

src/libutil/nar-accessor.cc

@@ -1,4 +1,5 @@
 #include "nix/util/nar-accessor.hh"
+#include "nix/util/file-descriptor.hh"
 #include "nix/util/archive.hh"
 
 #include <map>
@@ -281,7 +282,7 @@ GetNarBytes seekableGetNarBytes(const Path & path)
 GetNarBytes seekableGetNarBytes(Descriptor fd)
 {
     return [fd](uint64_t offset, uint64_t length) {
-        if (::lseek(fromDescriptorReadOnly(fd), offset, SEEK_SET) == -1)
+        if (lseek(fd, offset, SEEK_SET) == -1)
             throw SysError("seeking in file");
 
         std::string buf(length, 0);

src/libutil/serialise.cc

@@ -1,4 +1,5 @@
 #include "nix/util/serialise.hh"
+#include "nix/util/file-descriptor.hh"
 #include "nix/util/compression.hh"
 #include "nix/util/signals.hh"
 #include "nix/util/util.hh"
@@ -207,8 +208,7 @@ void FdSource::restart()
         throw Error("can't seek to the start of a file");
     buffer.reset();
     read = bufPosIn = bufPosOut = 0;
-    int fd_ = fromDescriptorReadOnly(fd);
-    if (lseek(fd_, 0, SEEK_SET) == -1)
+    if (lseek(fd, 0, SEEK_SET) == -1)
         throw SysError("seeking to the start of a file");
 }
 

src/libutil/windows/file-descriptor.cc

@@ -5,13 +5,12 @@
 #include "nix/util/windows-error.hh"
 #include "nix/util/file-path.hh"
 
-#ifdef _WIN32
-#  include <fileapi.h>
-#  include <error.h>
-#  include <namedpipeapi.h>
-#  include <namedpipeapi.h>
-#  define WIN32_LEAN_AND_MEAN
-#  include <windows.h>
+#include <fileapi.h>
+#include <error.h>
+#include <namedpipeapi.h>
+#include <namedpipeapi.h>
+#define WIN32_LEAN_AND_MEAN
+#include <windows.h>
 
 namespace nix {
 
@@ -46,16 +45,16 @@ void writeFull(HANDLE handle, std::string_view s, bool allowInterrupts)
         if (allowInterrupts)
             checkInterrupt();
         DWORD res;
-#  if _WIN32_WINNT >= 0x0600
+#if _WIN32_WINNT >= 0x0600
         auto path = handleToPath(handle); // debug; do it before because handleToPath changes lasterror
         if (!WriteFile(handle, s.data(), s.size(), &res, NULL)) {
             throw WinError("writing to file %1%:%2%", handle, path);
         }
-#  else
+#else
         if (!WriteFile(handle, s.data(), s.size(), &res, NULL)) {
             throw WinError("writing to file %1%", handle);
         }
-#  endif
+#endif
         if (res > 0)
             s.remove_prefix(res);
     }
@@ -120,7 +119,7 @@ void Pipe::create()
 
 //////////////////////////////////////////////////////////////////////
 
-#  if _WIN32_WINNT >= 0x0600
+#if _WIN32_WINNT >= 0x0600
 
 std::wstring windows::handleToFileName(HANDLE handle)
 {
@@ -149,7 +148,37 @@ Path windows::handleToPath(HANDLE handle)
     return os_string_to_string(handleToFileName(handle));
 }
 
-#  endif
+#endif
+
+off_t lseek(HANDLE h, off_t offset, int whence)
+{
+    DWORD method;
+    switch (whence) {
+    case SEEK_SET:
+        method = FILE_BEGIN;
+        break;
+    case SEEK_CUR:
+        method = FILE_CURRENT;
+        break;
+    case SEEK_END:
+        method = FILE_END;
+        break;
+    default:
+        throw Error("lseek: invalid whence %d", whence);
+    }
+
+    LARGE_INTEGER li;
+    li.QuadPart = offset;
+    LARGE_INTEGER newPos;
+
+    if (!SetFilePointerEx(h, li, &newPos, method)) {
+        /* Convert to a POSIX error, since caller code works with this as if it were
+           a POSIX lseek. */
+        errno = std::error_code(GetLastError(), std::system_category()).default_error_condition().value();
+        return -1;
+    }
+
+    return newPos.QuadPart;
+}
 
 } // namespace nix
-#endif