[Xen-devel] [PATCH 1/1] tools: reduce copies b/w ocaml Strings and Bytes

[Marcello Seri <marcello.seri@xxxxxxxxxx>]

When xenstore was ported to the new safe-string interface, it mostly
happened by making copyies of string into bytes and back.  The ideal
fix would be to rewrite all of the relevant interfaces to be uniformly
using bytes, but in the meanwhile we can improve the code by using unsafe
conversion functions (see
 
https://caml.inria.fr/pub/docs/manual-ocaml/libref/Bytes.html#3_Unsafeconversionsforadvancedusers).

In most cases we own the bytes that we are converting to string, or we
immediately make copies that we then mutate, or we use them immutably
as payloads for writes. In all these cases it is safe to use the unsafe
functions and prevent a copy.

This patch updates the code to use the unsafe conversions where possible.

Signed-off-by: Marcello Seri <marcello.seri@xxxxxxxxxx>
---
 tools/ocaml/libs/xb/xb.ml        | 4 +++-
 tools/ocaml/xenstored/logging.ml | 8 +++++---
 tools/ocaml/xenstored/stdext.ml  | 2 +-
 tools/ocaml/xenstored/utils.ml   | 6 +++---
 4 files changed, 12 insertions(+), 8 deletions(-)

diff --git a/tools/ocaml/libs/xb/xb.ml b/tools/ocaml/libs/xb/xb.ml
index 519842723b..660224f895 100644
--- a/tools/ocaml/libs/xb/xb.ml
+++ b/tools/ocaml/libs/xb/xb.ml
@@ -100,7 +100,9 @@ let write_mmap back con s len =
 
 let write con s len =
        match con.backend with
-       | Fd backfd     -> write_fd backfd con (Bytes.of_string s) len
+       (* we can use unsafe_of_string here as the bytes are used immutably
+          in the Unix.write operation. *)
+       | Fd backfd     -> write_fd backfd con (Bytes.unsafe_of_string s) len
        | Xenmmap backmmap -> write_mmap backmmap con s len
 
 (* NB: can throw Reconnect *)
diff --git a/tools/ocaml/xenstored/logging.ml b/tools/ocaml/xenstored/logging.ml
index e3c769fb2c..45a2c222e6 100644
--- a/tools/ocaml/xenstored/logging.ml
+++ b/tools/ocaml/xenstored/logging.ml
@@ -64,7 +64,7 @@ let truncate_line nb_chars line =
                Bytes.blit_string line 0 dst_line 0 (len - 2);
                Bytes.set dst_line (len-2) '.';
                Bytes.set dst_line (len-1) '.';
-               Bytes.to_string dst_line
+               Bytes.unsafe_to_string dst_line
        else line
 
 let log_rotate ref_ch log_file log_nb_files =
@@ -258,7 +258,7 @@ let sanitize_data data =
                if Bytes.get data i = '\000' then
                        Bytes.set data i ' '
        done;
-       String.escaped (Bytes.to_string data)
+       String.escaped (Bytes.unsafe_to_string data)
 
 let activate_access_log = ref true
 let access_log_destination = ref (File (Paths.xen_log_dir ^ 
"/xenstored-access.log"))
@@ -291,7 +291,9 @@ let access_logging ~con ~tid ?(data="") ~level access_type =
                                let date = string_of_date() in
                                let tid = string_of_tid ~con tid in
                                let access_type = string_of_access_type 
access_type in
-                               let data = sanitize_data (Bytes.of_string data) 
in
+                               (* we can use unsafe_of_string here as the 
sanitize_data function
+                                  immediately makes a copy of the data and 
operates on that. *)
+                               let data = sanitize_data 
(Bytes.unsafe_of_string data) in
                                let prefix = prefix !access_log_destination 
date in
                                let msg = Printf.sprintf "%s %s %s %s" prefix 
tid access_type data in
                                logger.write ~level msg)
diff --git a/tools/ocaml/xenstored/stdext.ml b/tools/ocaml/xenstored/stdext.ml
index 41411ee535..869fec36f2 100644
--- a/tools/ocaml/xenstored/stdext.ml
+++ b/tools/ocaml/xenstored/stdext.ml
@@ -122,7 +122,7 @@ let pidfile_write filename =
                let pid = Unix.getpid () in
                let buf = string_of_int pid ^ "\n" in
                let len = String.length buf in
-               if Unix.write fd (Bytes.of_string buf) 0 len <> len
+               if Unix.write fd (Bytes.unsafe_of_string buf) 0 len <> len
                then failwith "pidfile_write failed";
        )
        (fun () -> Unix.close fd)
diff --git a/tools/ocaml/xenstored/utils.ml b/tools/ocaml/xenstored/utils.ml
index 5fcb042351..73affb7ea4 100644
--- a/tools/ocaml/xenstored/utils.ml
+++ b/tools/ocaml/xenstored/utils.ml
@@ -52,7 +52,7 @@ let hexify s =
                Bytes.set hs (i * 2) seq.[0];
                Bytes.set hs (i * 2 + 1) seq.[1];
        done;
-       Bytes.to_string hs
+       Bytes.unsafe_to_string hs
 
 let unhexify hs =
        let char_of_hexseq seq0 seq1 = Char.chr (int_of_string (sprintf 
"0x%c%c" seq0 seq1)) in
@@ -61,7 +61,7 @@ let unhexify hs =
        do
                Bytes.set b i (char_of_hexseq hs.[i * 2] hs.[i * 2 + 1])
        done;
-       Bytes.to_string b
+       Bytes.unsafe_to_string b
 
 let trim_path path =
        try
@@ -87,7 +87,7 @@ let read_file_single_integer filename =
        let buf = Bytes.make 20 (char_of_int 0) in
        let sz = Unix.read fd buf 0 20 in
        Unix.close fd;
-       int_of_string (Bytes.to_string (Bytes.sub buf 0 sz))
+       int_of_string (Bytes.sub_string buf 0 sz)
 
 let path_complete path connection_path =
        if String.get path 0 <> '/' then
-- 
2.14.1


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