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_mspmfs.cpp

@@ -0,0 +1,172 @@
+#include "func_create_mspmfs.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 "fs_mkdir_as.hpp"
+#include <vector>
+#include <limits>
+
+
+#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::CreateMSPMFS::name() const
+{
+  return "mspmfs";
+}
+
+bool
+Func2::CreateMSPMFS::path_preserving() const
+{
+  return true;
+}
+
+
+int
+Func2::CreateMSPMFS::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;
+  fs::info_t info;
+  Branches::Ptr branches;
+  const Branch *chosen;
+  fs::path walk;
+  std::vector<fs::path> segments;
+  int fd;
+  fs::path fullpath;
+  mode_t mode;
+  FileInfo *fi;
+  mode_t dirmode;
+
+  branches = branches_;
+  chosen   = nullptr;
+  error    = ENOENT;
+  walk     = fusepath_.parent_path();
+
+  for(;;)
+    {
+      bool any;
+      u64 mfs;
+
+      any = false;
+      mfs = 0;
+      chosen = nullptr;
+
+      for(auto &branch : *branches)
+        {
+          if(branch.ro_or_nc())
+            error_and_continue(error,EROFS);
+          if(!fs::exists(branch.path,walk))
+            error_and_continue(error,ENOENT);
+          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);
+
+          any = true;
+          if(info.spaceavail < mfs)
+            continue;
+          mfs    = info.spaceavail;
+          chosen = &branch;
+        }
+
+      if(any)
+        break;
+      if(walk == "/")
+        break;
+      walk = walk.parent_path();
+    }
+
+  if(!chosen)
+    return -error;
+
+  for(fs::path p = fusepath_.parent_path();
+      !p.empty() && p != "/";
+      p = p.parent_path())
+    segments.push_back(p);
+
+  dirmode = (0777 & ~umask_);
+  for(auto it = segments.rbegin(); it != segments.rend(); ++it)
+    {
+      const Branch *src;
+      const fs::path full = chosen->path / *it;
+
+      if(fs::exists(full))
+        continue;
+
+      src = nullptr;
+      for(auto &b : *branches)
+        {
+          if(&b == chosen)
+            continue;
+          if(!fs::exists(b.path,*it))
+            continue;
+          src = &b;
+          break;
+        }
+
+      if(src != nullptr)
+        rv = fs::clonepath(src->path,chosen->path,*it);
+      else
+        rv = fs::mkdir_as(ugid_,full,dirmode);
+      if((rv < 0) && (rv != -EEXIST))
+        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;
+}