[lldb] Assorted improvements to the Pipe class (#128719)

The main motivation for this was the inconsistency in handling of
partial reads/writes between the windows and posix implementations
(windows was returning partial reads, posix was trying to fill the
buffer completely). I settle on the windows implementation, as that's
the more common behavior, and the "eager" version can be implemented on
top of that (in most cases, it isn't necessary, since we're writing just
a single byte).

Since this also required auditing the callers to make sure they're
handling partial reads/writes correctly, I used the opportunity to
modernize the function signatures as a forcing function. They now use
the `Timeout` class (basically an `optional<duration>`) to support both
polls (timeout=0) and blocking (timeout=nullopt) operations in a single
function, and use an `Expected` instead of a by-ref result to return the
number of bytes read/written.

As a drive-by, I also fix a problem with the windows implementation
where we were rounding the timeout value down, which meant that calls
could time out slightly sooner than expected.

1 files changed, 17 insertions, 7 deletions

diff --git a/lldb/source/Plugins/Process/gdb-remote/GDBRemoteCommunication.cpp b/lldb/source/Plugins/Process/gdb-remote/GDBRemoteCommunication.cpp
index f746ced..dad72a1 100644
--- a/lldb/source/Plugins/Process/gdb-remote/GDBRemoteCommunication.cpp
+++ b/lldb/source/Plugins/Process/gdb-remote/GDBRemoteCommunication.cpp
@@ -30,7 +30,9 @@
 #include "lldb/Utility/Log.h"
 #include "lldb/Utility/RegularExpression.h"
 #include "lldb/Utility/StreamString.h"
+#include "llvm/ADT/ArrayRef.h"
 #include "llvm/ADT/SmallString.h"
+#include "llvm/ADT/StringRef.h"
 #include "llvm/Config/llvm-config.h" // for LLVM_ENABLE_ZLIB
 #include "llvm/Support/ScopedPrinter.h"
 
@@ -1154,17 +1156,25 @@ Status GDBRemoteCommunication::StartDebugserverProcess(
       if (socket_pipe.CanWrite())
         socket_pipe.CloseWriteFileDescriptor();
       if (socket_pipe.CanRead()) {
-        // The port number may be up to "65535\0".
-        char port_cstr[6] = {0};
-        size_t num_bytes = sizeof(port_cstr);
         // Read port from pipe with 10 second timeout.
-        error = socket_pipe.ReadWithTimeout(
-            port_cstr, num_bytes, std::chrono::seconds{10}, num_bytes);
+        std::string port_str;
+        while (error.Success()) {
+          char buf[10];
+          if (llvm::Expected<size_t> num_bytes = socket_pipe.Read(
+                  buf, std::size(buf), std::chrono::seconds(10))) {
+            if (*num_bytes == 0)
+              break;
+            port_str.append(buf, *num_bytes);
+          } else {
+            error = Status::FromError(num_bytes.takeError());
+          }
+        }
         if (error.Success() && (port != nullptr)) {
-          assert(num_bytes > 0 && port_cstr[num_bytes - 1] == '\0');
+          // NB: Deliberately using .c_str() to stop at embedded '\0's
+          llvm::StringRef port_ref = port_str.c_str();
           uint16_t child_port = 0;
           // FIXME: improve error handling
-          llvm::to_integer(port_cstr, child_port);
+          llvm::to_integer(port_ref, child_port);
           if (*port == 0 || *port == child_port) {
             *port = child_port;
             LLDB_LOGF(log,