Allow character devices to block write attempts until there is more space.

Author: https://github.com/awesomekling
Commit: https://github.com/SerenityOS/serenity/commit/e452303c664

Kernel/Console.h

@@ -19,6 +19,7 @@ public:
     virtual ~Console() override;
 
     virtual bool has_data_available_for_reading(Process&) const override;
+    virtual bool can_write(Process&) const override { return true; }
     virtual ssize_t read(byte* buffer, size_t size) override;
     virtual ssize_t write(const byte* data, size_t size) override;
 

Kernel/DoubleBuffer.h

@@ -16,6 +16,8 @@ public:
 
     bool is_empty() const { return m_read_buffer_index >= m_read_buffer->size() && m_write_buffer->is_empty(); }
 
+    size_t bytes_in_write_buffer() const { return m_write_buffer->size(); }
+
 private:
     void flip();
 

Kernel/FIFO.cpp

@@ -51,7 +51,7 @@ bool FIFO::can_read() const
 
 bool FIFO::can_write() const
 {
-    return true;
+    return m_buffer.bytes_in_write_buffer() < 4096;
 }
 
 ssize_t FIFO::read(byte* buffer, size_t size)

Kernel/Keyboard.h

@@ -40,6 +40,7 @@ private:
     virtual ssize_t read(byte* buffer, size_t) override;
     virtual ssize_t write(const byte* buffer, size_t) override;
     virtual bool has_data_available_for_reading(Process&) const override;
+    virtual bool can_write(Process&) const override { return true; }
 
     void emit(byte);
 

Kernel/MasterPTY.cpp

@@ -36,6 +36,11 @@ bool MasterPTY::has_data_available_for_reading(Process&) const
     return !m_buffer.is_empty();
 }
 
+bool MasterPTY::can_write(Process&) const
+{
+    return m_buffer.bytes_in_write_buffer() < 4096;
+}
+
 void MasterPTY::on_slave_write(const byte* data, size_t size)
 {
     m_buffer.write(data, size);

Kernel/MasterPTY.h

@@ -14,6 +14,7 @@ public:
     virtual ssize_t read(byte*, size_t) override;
     virtual ssize_t write(const byte*, size_t) override;
     virtual bool has_data_available_for_reading(Process&) const override;
+    virtual bool can_write(Process&) const override;
     virtual bool is_master_pty() const override { return true; }
 
     String pts_name() const;

Kernel/PS2MouseDevice.h

@@ -15,6 +15,7 @@ public:
     virtual bool has_data_available_for_reading(Process&) const override;
     virtual ssize_t read(byte* buffer, size_t) override;
     virtual ssize_t write(const byte* buffer, size_t) override;
+    virtual bool can_write(Process&) const override { return true; }
 
 private:
     // ^IRQHandler

Kernel/Process.cpp

@@ -1023,7 +1023,7 @@ ssize_t Process::sys$write(int fd, const void* data, size_t size)
 #ifdef IO_DEBUG
             dbgprintf("while %u < %u\n", nwritten, size);
 #endif
-            if (!descriptor->can_write()) {
+            if (!descriptor->can_write(*this)) {
 #ifdef IO_DEBUG
                 dbgprintf("block write on %d\n", fd);
 #endif

Kernel/Scheduler.cpp

@@ -59,7 +59,7 @@ bool Scheduler::pick_next()
 
         if (process.state() == Process::BlockedWrite) {
             ASSERT(process.m_blocked_fd != -1);
-            if (process.m_fds[process.m_blocked_fd].descriptor->can_write())
+            if (process.m_fds[process.m_blocked_fd].descriptor->can_write(process))
                 process.unblock();
             return true;
         }

Kernel/SlavePTY.cpp

@@ -29,3 +29,8 @@ void SlavePTY::on_tty_write(const byte* data, size_t size)
 {
     m_master->on_slave_write(data, size);
 }
+
+bool SlavePTY::can_write(Process& process) const
+{
+    return m_master->can_write(process);
+}

Kernel/SlavePTY.h

@@ -16,6 +16,7 @@ public:
 
 protected:
     virtual void on_tty_write(const byte*, size_t) override;
+    virtual bool can_write(Process&) const override;
 
 private:
     unsigned m_index;

Kernel/TTY.cpp

@@ -47,6 +47,11 @@ bool TTY::has_data_available_for_reading(Process&) const
     return !m_buffer.is_empty();
 }
 
+bool TTY::can_write(Process&) const
+{
+    return true;
+}
+
 void TTY::emit(byte ch)
 {
     if (should_generate_signals()) {

Kernel/TTY.h

@@ -13,6 +13,7 @@ public:
     virtual ssize_t read(byte*, size_t) override;
     virtual ssize_t write(const byte*, size_t) override;
     virtual bool has_data_available_for_reading(Process&) const override;
+    virtual bool can_write(Process&) const override;
     virtual int ioctl(Process&, unsigned request, unsigned arg) override final;
 
     virtual String tty_name() const = 0;

VirtualFileSystem/CharacterDevice.h

@@ -13,6 +13,7 @@ public:
     RetainPtr<FileDescriptor> open(int options);
 
     virtual bool has_data_available_for_reading(Process&) const = 0;
+    virtual bool can_write(Process&) const = 0;
 
     virtual ssize_t read(byte* buffer, size_t bufferSize) = 0;
     virtual ssize_t write(const byte* buffer, size_t bufferSize) = 0;

VirtualFileSystem/FileDescriptor.cpp

@@ -165,12 +165,14 @@ ssize_t FileDescriptor::write(const byte* data, size_t size)
     return -1;
 }
 
-bool FileDescriptor::can_write()
+bool FileDescriptor::can_write(Process& process)
 {
     if (is_fifo()) {
         ASSERT(fifo_direction() == FIFO::Writer);
         return m_fifo->can_write();
     }
+    if (m_vnode->isCharacterDevice())
+        return m_vnode->characterDevice()->can_write(process);
     return true;
 }
 

VirtualFileSystem/FileDescriptor.h

@@ -30,7 +30,7 @@ public:
     int stat(Unix::stat*);
 
     bool has_data_available_for_reading(Process&);
-    bool can_write();
+    bool can_write(Process&);
 
     ssize_t get_dir_entries(byte* buffer, size_t);
 

VirtualFileSystem/FullDevice.h

@@ -6,10 +6,11 @@ class FullDevice final : public CharacterDevice {
     AK_MAKE_ETERNAL
 public:
     FullDevice();
-    virtual ~FullDevice();
+    virtual ~FullDevice() override;
 
     virtual ssize_t read(byte* buffer, size_t bufferSize) override;
     virtual ssize_t write(const byte* buffer, size_t bufferSize) override;
     virtual bool has_data_available_for_reading(Process&) const override;
+    virtual bool can_write(Process&) const override { return true; }
 };
 

VirtualFileSystem/NullDevice.h

@@ -10,6 +10,7 @@ public:
 
     virtual ssize_t read(byte* buffer, size_t bufferSize) override;
     virtual ssize_t write(const byte* buffer, size_t bufferSize) override;
+    virtual bool can_write(Process&) const override { return true; }
     virtual bool has_data_available_for_reading(Process&) const override;
 };
 

VirtualFileSystem/RandomDevice.h

@@ -11,5 +11,6 @@ public:
     virtual ssize_t read(byte* buffer, size_t bufferSize) override;
     virtual ssize_t write(const byte* buffer, size_t bufferSize) override;
     virtual bool has_data_available_for_reading(Process&) const override;
+    virtual bool can_write(Process&) const override { return true; }
 };
 

VirtualFileSystem/ZeroDevice.h

@@ -11,5 +11,6 @@ public:
     virtual ssize_t read(byte* buffer, size_t bufferSize) override;
     virtual ssize_t write(const byte* buffer, size_t bufferSize) override;
     virtual bool has_data_available_for_reading(Process&) const override;
+    virtual bool can_write(Process&) const override { return true; }
 };
 

Widgets/GUIEventDevice.h

@@ -8,7 +8,9 @@ public:
     virtual ~GUIEventDevice() override;
 
 private:
-    virtual bool has_data_available_for_reading(Process&) const override;
+    // ^CharacterDevice
     virtual ssize_t read(byte* buffer, size_t bufferSize) override;
     virtual ssize_t write(const byte* buffer, size_t bufferSize) override;
+    virtual bool has_data_available_for_reading(Process&) const override;
+    virtual bool can_write(Process&) const override { return true; }
 };