ENH: allow null pointer when reading binary block

- have read(nullptr, count) and readRaw(nullptr, count) act like a
  forward seek instead of failing.
  This lets it be used to advance through a file without needing to
  allocate (and discard) storage space etc.

src/OpenFOAM/db/IOstreams/IOstreams/Istream.H

@@ -6,7 +6,7 @@
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
     Copyright (C) 2011-2016 OpenFOAM Foundation
-    Copyright (C) 2017-2022 OpenCFD Ltd.
+    Copyright (C) 2017-2023 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -161,8 +161,10 @@ public:
             //- Read a double
             virtual Istream& read(double&) = 0;
 
-            //- Read binary block
-            virtual Istream& read(char*, std::streamsize) = 0;
+            //- Read binary block (with any possible block delimiters).
+            //- Reading into a null pointer shall ideally behave like a seek
+            //- operation.
+            virtual Istream& read(char* data, std::streamsize count) = 0;
 
             //- Start of low-level raw binary read
             virtual bool beginRawRead() = 0;
@@ -170,8 +172,10 @@ public:
             //- End of low-level raw binary read
             virtual bool endRawRead() = 0;
 
-            //- Low-level raw binary read
-            virtual Istream& readRaw(char*, std::streamsize) = 0;
+            //- Low-level raw binary read (without possible block delimiters).
+            //- Reading into a null pointer shall ideally behave like a seek
+            //- operation.
+            virtual Istream& readRaw(char* data, std::streamsize count) = 0;
 
             //- Rewind the stream so that it may be read again
             virtual void rewind() = 0;

src/OpenFOAM/db/IOstreams/Pstreams/UIPstream.H

@@ -75,6 +75,7 @@ class UIPstreamBase
 
         //- Read count bytes of data from the receive buffer.
         //  Prior data alignment is done by prepareBuffer
+        //  Reading into a null pointer behaves like a forward seek
         inline void readFromBuffer(void* data, const size_t count);
 
         //- Read string length and string content
@@ -181,9 +182,13 @@ public:
             Istream& read(double& val);
 
             //- Read binary block with 8-byte alignment.
+            //- Reading into a null pointer behaves like a forward seek of
+            //- count characters.
             Istream& read(char* data, std::streamsize count);
 
-            //- Low-level raw binary read
+            //- Low-level raw binary read.
+            //- Reading into a null pointer behaves like a forward seek of
+            //- count characters.
             Istream& readRaw(char* data, std::streamsize count);
 
             //- Start of low-level raw binary read

src/OpenFOAM/db/IOstreams/Pstreams/UIPstreamBase.C

@@ -106,12 +106,15 @@ inline void Foam::UIPstreamBase::readFromBuffer
     const size_t count
 )
 {
-    const char* const __restrict__ buf = &recvBuf_[recvBufPos_];
-    char* const __restrict__ output = reinterpret_cast<char*>(data);
-
-    for (size_t i = 0; i < count; ++i)
+    if (data)
     {
-        output[i] = buf[i];
+        const char* const __restrict__ buf = &recvBuf_[recvBufPos_];
+        char* const __restrict__ output = reinterpret_cast<char*>(data);
+
+        for (size_t i = 0; i < count; ++i)
+        {
+            output[i] = buf[i];
+        }
     }
 
     recvBufPos_ += count;

src/OpenFOAM/db/IOstreams/Sstreams/ISstream.C

@@ -6,7 +6,7 @@
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
     Copyright (C) 2011-2016 OpenFOAM Foundation
-    Copyright (C) 2017-2022 OpenCFD Ltd.
+    Copyright (C) 2017-2023 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -1018,19 +1018,41 @@ Foam::Istream& Foam::ISstream::read(double& val)
 }
 
 
-Foam::Istream& Foam::ISstream::read(char* buf, std::streamsize count)
+Foam::Istream& Foam::ISstream::read(char* data, std::streamsize count)
 {
     beginRawRead();
-    readRaw(buf, count);
+    readRaw(data, count);
     endRawRead();
 
     return *this;
 }
 
 
-Foam::Istream& Foam::ISstream::readRaw(char* buf, std::streamsize count)
+Foam::Istream& Foam::ISstream::readRaw(char* data, std::streamsize count)
 {
-    is_.read(buf, count);
+    if (count)
+    {
+        if (data)
+        {
+            is_.read(data, count);
+        }
+        else
+        {
+            // Forward seek
+            is_.seekg(is_.tellg() + count);
+
+            // Not sure if this is needed (as per rewind)
+            // some documentation indicates that ifstream needs
+            // seekg with values from a tellg
+            //
+            // stdStream().rdbuf()->pubseekpos
+            // (
+            //     count,
+            //     std::ios_base::seekdir::cur,
+            //     std::ios_base::in
+            // );
+        }
+    }
     syncState();
     return *this;
 }

src/OpenFOAM/db/IOstreams/Sstreams/ISstream.H

@@ -222,10 +222,14 @@ public:
             //- Read a double
             virtual Istream& read(double& val);
 
-            //- Read binary block
-            virtual Istream& read(char* buf, std::streamsize count);
+            //- Read binary block (with any possible block delimiters).
+            //- Reading into a null pointer behaves like a forward seek of
+            //- count characters.
+            virtual Istream& read(char* data, std::streamsize count);
 
-            //- Low-level raw binary read
+            //- Low-level raw binary read (without possible block delimiters).
+            //- Reading into a null pointer behaves like a forward seek of
+            //- count characters.
             virtual Istream& readRaw(char* data, std::streamsize count);
 
             //- Start of low-level raw binary read