Implement bespoke branch policies

Consolidate branch policy selection behind the BranchPolicy interfaces and update filesystem operations to use the bespoke policy implementations.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/func_create_pfrd.cpp

@@ -0,0 +1,159 @@
+#include "func_create_pfrd.hpp"
+
+#include "errno.hpp"
+#include "fs_clonepath.hpp"
+#include "fs_exists.hpp"
+#include "fs_info.hpp"
+#include "fs_info_t.hpp"
+#include "fs_path.hpp"
+#include "fileinfo.hpp"
+#include "fs_acl.hpp"
+#include "fs_open_as.hpp"
+#include "rnd.hpp"
+#include <vector>
+
+
+#define error_and_continue(CUR,ERR)             \
+  do {                                          \
+    ::_calc_error(CUR,ERR);                     \
+    continue;                                   \
+  } while(0)
+
+static
+inline
+void
+_calc_error(int &cur_,
+            const int err_)
+{
+  switch(cur_)
+    {
+    default:
+    case ENOENT:
+      cur_ = err_;
+      break;
+    case ENOSPC:
+      if(err_ != ENOENT)
+        cur_ = err_;
+      break;
+    case EROFS:
+      if((err_ != ENOENT) && (err_ != ENOSPC))
+        cur_ = err_;
+      break;
+    }
+}
+
+
+std::string_view
+Func2::CreatePFRD::name() const
+{
+  return "pfrd";
+}
+
+bool
+Func2::CreatePFRD::path_preserving() const
+{
+  return false;
+}
+
+
+int
+Func2::CreatePFRD::operator()(const ugid_t      &ugid_,
+                              const Branches    &branches_,
+                              const fs::path    &fusepath_,
+                              fuse_file_info_t *&ffi_,
+                              const mode_t      &mode_,
+                              const mode_t      &umask_)
+{
+  int rv;
+  int error;
+  u64 sum;
+  fs::info_t info;
+  Branches::Ptr branches;
+  const Branch *chosen;
+  const Branch *clone_src;
+  struct W { Branch *b; u64 weight; };
+  std::vector<W> writable;
+  int fd;
+  fs::path fullpath;
+  mode_t mode;
+  FileInfo *fi;
+
+  branches  = branches_;
+  chosen    = nullptr;
+  clone_src = nullptr;
+  error     = ENOENT;
+  sum       = 0;
+  for(auto &branch : *branches)
+    {
+      if(branch.ro_or_nc())
+        error_and_continue(error,EROFS);
+      rv = fs::info(branch.path,&info);
+      if(rv < 0)
+        error_and_continue(error,ENOENT);
+      if(info.readonly)
+        error_and_continue(error,EROFS);
+      if(info.spaceavail < branch.minfreespace())
+        error_and_continue(error,ENOSPC);
+
+      writable.push_back({&branch,info.spaceavail});
+      sum += info.spaceavail;
+    }
+
+  if(writable.empty())
+    return -error;
+  if(sum == 0)
+    return -ENOSPC;
+
+  {
+    u64 idx;
+    u64 threshold;
+
+    idx = 0;
+    threshold = RND::rand64(sum);
+    for(const auto &w : writable)
+      {
+        if(w.weight == 0)
+          continue;
+        idx += w.weight;
+        if(idx > threshold)
+          {
+            chosen = w.b;
+            break;
+          }
+      }
+    if(!chosen)
+      chosen = writable.back().b;
+  }
+
+  for(auto &branch : *branches)
+    {
+      if(!fs::exists(branch.path,fusepath_.parent_path()))
+        continue;
+      clone_src = &branch;
+      break;
+    }
+
+  if(!clone_src)
+    return -ENOENT;
+
+  if(clone_src != chosen)
+    {
+      rv = fs::clonepath(clone_src->path,chosen->path,fusepath_.parent_path());
+      if(rv < 0)
+        return rv;
+    }
+
+  fullpath = chosen->path / fusepath_;
+  mode = mode_;
+  if(!fs::acl::dir_has_defaults(fullpath))
+    mode &= ~umask_;
+
+  fd = fs::open_as(ugid_,fullpath,ffi_->flags,mode);
+  if(fd < 0)
+    return fd;
+
+  fi = new FileInfo(fd,chosen,fusepath_,ffi_->direct_io);
+  ffi_->fh = fi->to_fh();
+
+  return 0;
+}