Make wxFile::ReadAll() work for unseekable files too

Calling this function with an unseekable file, such as any file under
/sys on Linux systems, would previously just hang as the loop condition
was never satisfied when length was -1.

Fix this by checking for this case and using a different approach by
extending the buffer we read the data into as we go instead of
preallocating it all at once.

See #9965.

docs/changes.txt

@@ -106,6 +106,7 @@ All:
 - Add UTF-8 support to wxZipOutputStream (Tobias Taschner).
 - Update all bundled 3rd party libraries to their latest versions.
 - Use unique prefix for all zlib symbols to avoid link conflicts.
+- Make wxFile::ReadAll() work for unseekable files too.
 
 All (GUI):
 

interface/wx/file.h

@@ -377,6 +377,8 @@ public:
     /**
         Reads the entire contents of the file into a string.
 
+        Since wxWidgets 3.1.1 this method also works for unseekable files.
+
         @param str
             Non-@NULL pointer to a string to read data into.
         @param conv

src/common/file.cpp

@@ -261,24 +261,71 @@ bool wxFile::ReadAll(wxString *str, const wxMBConv& conv)
 {
     wxCHECK_MSG( str, false, wxS("Output string must be non-NULL") );
 
+    static const ssize_t READSIZE = 4096;
+
+    wxCharBuffer buf;
+
     ssize_t length = Length();
+    if ( length != -1 )
+    {
         wxCHECK_MSG( (wxFileOffset)length == Length(), false, wxT("huge file not supported") );
 
-    wxCharBuffer buf(length);
+        if ( !buf.extend(length) )
+            return false;
+
         char* p = buf.data();
         for ( ;; )
         {
-        static const ssize_t READSIZE = 4096;
-
             ssize_t nread = Read(p, length > READSIZE ? READSIZE : length);
             if ( nread == wxInvalidOffset )
                 return false;
 
-        p += nread;
-        if ( length <= nread )
+            if ( nread == 0 )
+            {
+                // We have reached EOF before reading the entire length of the
+                // file. This can happen for some special files (e.g. those
+                // under /sys on Linux systems) or even for regular files if
+                // another process has truncated the file since we started
+                // reading it, so deal with it gracefully.
+                buf.shrink(p - buf.data());
                 break;
+            }
 
+            p += nread;
             length -= nread;
+
+            if ( !length )
+            {
+                // Notice that we don't keep reading after getting the expected
+                // number of bytes, even though in principle a situation
+                // similar to the one described above, with another process
+                // extending the file since we started to read it, is possible.
+                // But returning just the data that was in the file when we
+                // originally started reading it isn't really wrong in this
+                // case, so keep things simple and just do it like this.
+                break;
+            }
+        }
+    }
+    else // File is not seekable
+    {
+        for ( ;; )
+        {
+            const size_t len = buf.length();
+            if ( !buf.extend(len + READSIZE) )
+                return false;
+
+            ssize_t nread = Read(buf.data() + len, READSIZE);
+            if ( nread == wxInvalidOffset )
+                return false;
+
+            if ( nread < READSIZE )
+            {
+                // We have reached EOF.
+                buf.shrink(len + nread);
+                break;
+            }
+        }
     }
 
     str->assign(buf, conv);

tests/file/filetest.cpp

@@ -139,4 +139,28 @@ void FileTestCase::TempFile()
     CPPUNIT_ASSERT( wxRemoveFile(wxT("test2")) );
 }
 
+#ifdef __LINUX__
+
+// Check that GetSize() works correctly for special files.
+TEST_CASE("wxFile::Special", "[file][linux][special-file]")
+{
+    // This file is not seekable and has 0 size, but can still be read.
+    wxFile fileProc("/proc/diskstats");
+    CHECK( fileProc.IsOpened() );
+
+    wxString s;
+    CHECK( fileProc.ReadAll(&s) );
+    CHECK( !s.empty() );
+
+    // All files in /sys seem to have size of 4KiB currently, even if they
+    // don't have that much data in them.
+    wxFile fileSys("/sys/power/state");
+    CHECK( fileSys.IsOpened() );
+    CHECK( fileSys.ReadAll(&s) );
+    CHECK( !s.empty() );
+    CHECK( s.length() < 4096 );
+}
+
+#endif // __LINUX__
+
 #endif // wxUSE_FILE