clear error_code on successful file operations

Fixes #3035

include/boost/beast/core/detail/win32_unicode_path.hpp

@@ -29,6 +29,7 @@ class win32_unicode_path
 
 public:
     win32_unicode_path(const char* utf8_path, error_code& ec) {
+        ec = {};
         int ret = mb2wide(utf8_path, static_buf_.data(),
             static_buf_.size());
         if (ret == 0)

include/boost/beast/core/impl/file_posix.ipp

@@ -288,12 +288,13 @@ read(void* buffer, std::size_t n, error_code& ec) const
         if(result == 0)
         {
             // short read
-            return nread;
+            break;
         }
         n -= result;
         nread += result;
         buffer = static_cast<char*>(buffer) + result;
     }
+    ec = {};
     return nread;
 }
 
@@ -328,6 +329,7 @@ write(void const* buffer, std::size_t n, error_code& ec)
         nwritten += result;
         buffer = static_cast<char const*>(buffer) + result;
     }
+    ec = {};
     return nwritten;
 }
 

include/boost/beast/core/impl/file_stdio.ipp

@@ -300,6 +300,7 @@ read(void* buffer, std::size_t n, error_code& ec) const
         ec.assign(errno, generic_category());
         return 0;
     }
+    ec = {};
     return nread;
 }
 
@@ -318,6 +319,7 @@ write(void const* buffer, std::size_t n, error_code& ec)
         ec.assign(errno, generic_category());
         return 0;
     }
+    ec = {};
     return nwritten;
 }
 

include/boost/beast/core/impl/file_win32.ipp

@@ -317,7 +317,10 @@ read(void* buffer, std::size_t n, error_code& ec)
             return nread;
         }
         if(bytesRead == 0)
+        {
+            ec = {};
             return nread;
+        }
         n -= bytesRead;
         nread += bytesRead;
         buffer = static_cast<char*>(buffer) + bytesRead;
@@ -357,7 +360,10 @@ write(void const* buffer, std::size_t n, error_code& ec)
             return nwritten;
         }
         if(bytesWritten == 0)
+        {
+            ec = {};
             return nwritten;
+        }
         n -= bytesWritten;
         nwritten += bytesWritten;
         buffer = static_cast<char const*>(buffer) + bytesWritten;

test/beast/core/file_test.hpp

@@ -444,6 +444,87 @@ test_file()
         remove(path);
     }
 
+    // https://github.com/boostorg/beast/issues/3035
+    //
+    // A successful operation must clear `ec`.
+    {
+        string_view const s = "Hello, world!";
+
+        // write
+        {
+            File f;
+            {
+                error_code ec = error::timeout;
+                f.open(path, file_mode::write, ec);
+                BEAST_EXPECTS(! ec, ec.message());
+            }
+            {
+                error_code ec = error::timeout;
+                f.write(s.data(), s.size(), ec);
+                BEAST_EXPECTS(! ec, ec.message());
+            }
+            {
+                error_code ec = error::timeout;
+                f.write(s.data(), 0, ec);
+                BEAST_EXPECTS(! ec, ec.message());
+            }
+            {
+                error_code ec = error::timeout;
+                auto n = f.size(ec);
+                BEAST_EXPECTS(! ec, ec.message());
+                BEAST_EXPECT(n == s.size());
+            }
+            {
+                error_code ec = error::timeout;
+                f.pos(ec);
+                BEAST_EXPECTS(! ec, ec.message());
+            }
+            {
+                error_code ec = error::timeout;
+                f.seek(0, ec);
+                BEAST_EXPECTS(! ec, ec.message());
+            }
+            {
+                error_code ec = error::timeout;
+                f.close(ec);
+                BEAST_EXPECTS(! ec, ec.message());
+            }
+        }
+
+        // read
+        {
+            File f;
+            {
+                error_code ec = error::timeout;
+                f.open(path, file_mode::read, ec);
+                BEAST_EXPECTS(! ec, ec.message());
+            }
+            {
+                std::string buf;
+                buf.resize(s.size());
+                error_code ec = error::timeout;
+                auto n = f.read(&buf[0], buf.size(), ec);
+                BEAST_EXPECTS(! ec, ec.message());
+                BEAST_EXPECT(n == s.size());
+                BEAST_EXPECT(buf == s);
+            }
+            {
+                char c;
+                error_code ec = error::timeout;
+                f.read(&c, 0, ec);
+                BEAST_EXPECTS(! ec, ec.message());
+            }
+            {
+                char c;
+                error_code ec = error::timeout;
+                auto n = f.read(&c, 1, ec);
+                BEAST_EXPECTS(! ec, ec.message());
+                BEAST_EXPECT(n == 0);
+            }
+        }
+        remove(path);
+    }
+
     BEAST_EXPECT(! fs::exists(path));
 }