io/FileDescriptor: pass std::span to ReadAt()

src/bp/EmulateModAuthEasy.cxx

@@ -114,7 +114,7 @@ ParseBasicAuth(const char *authorization) noexcept
 static char *
 ReadFirstLine(FileDescriptor fd, char *buffer, size_t size) noexcept
 {
-	ssize_t nbytes = fd.ReadAt(0, buffer, size - 1);
+	ssize_t nbytes = fd.ReadAt(0, std::as_writable_bytes(std::span{buffer, size - 1}));
 	if (nbytes <= 0)
 		return nullptr;
 

src/io/Buffered.cxx

@@ -39,10 +39,10 @@ ReadToBufferAt(FileDescriptor fd, off_t offset,
 	if (w.empty())
 		return -2;
 
-	if (length > w.size())
-		length = w.size();
+	if (w.size() > length)
+		w = w.first(length);
 
-	ssize_t nbytes = fd.ReadAt(offset, w.data(), length);
+	ssize_t nbytes = fd.ReadAt(offset, w);
 	if (nbytes > 0)
 		buffer.Append((size_t)nbytes);
 

src/istream/BufferedIstream.cxx

@@ -153,16 +153,19 @@ BufferedIstream::ReadToBuffer(FileDescriptor fd, off_t offset,
 	if (!buffer.IsDefined())
 		buffer = fb_pool_get().Alloc();
 
-	const auto w = buffer.Write();
+	auto w = buffer.Write();
 	if (w.empty())
 		/* buffer is full - the "ready" call is pending */
 		return IstreamDirectResult::BLOCKING;
 
-	const std::size_t n_try = std::min(w.size(), max_length);
+	const std::size_t buffer_space = w.size();
+
+	if (w.size() > max_length)
+		w = w.first(max_length);
 
 	ssize_t nbytes = HasOffset(offset)
-		? fd.ReadAt(offset, w.data(), n_try)
-		: fd.Read(w.data(), n_try);
+		? fd.ReadAt(offset, w)
+		: fd.Read(w);
 	if (nbytes <= 0)
 		return nbytes < 0
 			? IstreamDirectResult::ERRNO
@@ -171,7 +174,7 @@ BufferedIstream::ReadToBuffer(FileDescriptor fd, off_t offset,
 	input.ConsumeDirect(nbytes);
 	buffer.Append(nbytes);
 
-	if ((std::size_t)nbytes == w.size())
+	if ((std::size_t)nbytes == buffer_space)
 		/* buffer has become full - we can report to handler */
 		defer_ready.Schedule();
 

src/istream/FileIstream.cxx

@@ -142,7 +142,7 @@ FileIstream::TryData()
 	if (GetRemaining() < off_t(w.size()))
 		w = w.first(GetRemaining());
 
-	ssize_t nbytes = fd.ReadAt(offset, w.data(), w.size());
+	ssize_t nbytes = fd.ReadAt(offset, w);
 	if (nbytes == 0) {
 		throw FmtRuntimeError("premature end of file in '{}'", path);
 	} else if (nbytes == -1) {

src/memory/SinkGrowingBuffer.cxx

@@ -6,8 +6,6 @@
 #include "istream/Bucket.hxx"
 #include "io/FileDescriptor.hxx"
 
-#include <algorithm>
-
 IstreamReadyResult
 GrowingBufferSink::OnIstreamReady() noexcept
 {
@@ -60,11 +58,12 @@ GrowingBufferSink::OnDirect(FdType, FileDescriptor fd, off_t offset,
 			    [[maybe_unused]] bool then_eof) noexcept
 {
 	auto w = buffer.BeginWrite();
-	const std::size_t n = std::min(w.size(), max_length);
+	if (w.size() > max_length)
+		w = w.first(max_length);
 
 	ssize_t nbytes = HasOffset(offset)
-		? fd.ReadAt(offset, w.data(), n)
-		: fd.Read(w.data(), n);
+		? fd.ReadAt(offset, w)
+		: fd.Read(w);
 	if (nbytes <= 0)
 		return nbytes < 0
 			? IstreamDirectResult::ERRNO

src/memory/sink_rubber.cxx

@@ -71,7 +71,7 @@ fd_read(FdType type, FileDescriptor fd, off_t offset,
 	return IsAnySocket(type)
 		? SocketDescriptor::FromFileDescriptor(fd).ReadNoWait(dest)
 		: (IstreamHandler::HasOffset(offset)
-		   ? fd.ReadAt(offset, dest.data(), dest.size())
+		   ? fd.ReadAt(offset, dest)
 		   : fd.Read(dest));
 }
 

test/t_cgi.cxx

@@ -112,13 +112,14 @@ Context::OnDirect(FdType, FileDescriptor fd, off_t offset,
 		return IstreamDirectResult::BLOCKING;
 	}
 
-	char buffer[256];
-	if (max_length > sizeof(buffer))
-		max_length = sizeof(buffer);
+	std::byte buffer[256];
+	std::span<std::byte> w{buffer};
+	if (w.size() > max_length)
+		w = w.first(max_length);
 
 	ssize_t nbytes = HasOffset(offset)
-		? fd.ReadAt(offset, buffer, max_length)
-		: fd.Read(buffer, max_length);
+		? fd.ReadAt(offset, w)
+		: fd.Read(w);
 	if (nbytes <= 0)
 		return nbytes < 0
 			? IstreamDirectResult::ERRNO