fix: open files with read modifier

The commit
7ee43cc
("fix: Support creating file with open_file_nonblock")
did modify the file opening utility function by adding
`open` option, but it also removed the `read` option from it.
This causes an error during metrics and logs file initialization code if
the the file is a FIFO and there are no readers already reading from it.
This is because `open` returns `ENXIO` when opening a FIFO write-only as
described in the man page:
```
ENXIO  O_NONBLOCK | O_WRONLY is set, the named file is a FIFO,  and  no
             process has the FIFO open for reading.
```
Fix is just a partial revert of the part that changed the file opening
logic by re-introducing same `open_file_nonblock` as it was before but
with added `create` flag.

Signed-off-by: Egor Lazarchuk <yegorlz@amazon.co.uk>

Author: ShadowCurse

src/vmm/src/device_manager/mod.rs

@@ -35,7 +35,7 @@ use crate::devices::virtio::device::VirtioDevice;
 use crate::devices::virtio::transport::mmio::{IrqTrigger, MmioTransport};
 use crate::resources::VmResources;
 use crate::snapshot::Persist;
-use crate::utils::open_file_write_nonblock;
+use crate::utils::open_file_nonblock;
 use crate::vstate::bus::BusError;
 use crate::vstate::memory::GuestMemoryMmap;
 use crate::{EmulateSerialInitError, EventManager, Vm};
@@ -125,7 +125,7 @@ impl DeviceManager {
         output: Option<&PathBuf>,
     ) -> Result<Arc<Mutex<SerialDevice>>, std::io::Error> {
         let (serial_in, serial_out) = match output {
-            Some(path) => (None, open_file_write_nonblock(path).map(SerialOut::File)?),
+            Some(path) => (None, open_file_nonblock(path).map(SerialOut::File)?),
             None => {
                 Self::set_stdout_nonblocking();
 

src/vmm/src/logger/logging.rs

@@ -13,7 +13,7 @@ use serde::{Deserialize, Deserializer, Serialize};
 use utils::time::LocalTime;
 
 use super::metrics::{IncMetric, METRICS};
-use crate::utils::open_file_write_nonblock;
+use crate::utils::open_file_nonblock;
 
 /// Default level filter for logger matching the swagger specification
 /// (`src/firecracker/swagger/firecracker.yaml`).
@@ -62,7 +62,7 @@ impl Logger {
         );
 
         if let Some(log_path) = config.log_path {
-            let file = open_file_write_nonblock(&log_path).map_err(LoggerUpdateError)?;
+            let file = open_file_nonblock(&log_path).map_err(LoggerUpdateError)?;
 
             guard.target = Some(file);
         };

src/vmm/src/utils/mod.rs

@@ -76,14 +76,16 @@ pub const fn align_down(addr: u64, align: u64) -> u64 {
     addr & !(align - 1)
 }
 
-/// Create and open a File for writing to it.
-/// In case we open a FIFO, in order to not block the instance if nobody is consuming the message
-/// that is flushed to it, we are opening it with `O_NONBLOCK` flag.
-/// In this case, writing to a pipe will start failing when reaching 64K of unconsumed content.
-pub fn open_file_write_nonblock(path: &Path) -> Result<File, std::io::Error> {
+/// Create and open a file for both reading and writing to it with a O_NONBLOCK flag.
+/// In case we open a FIFO, we need all READ, WRITE and O_NONBLOCK in order to not block the process
+/// if nobody is consuming the message. Otherwise opening the FIFO with only WRITE and O_NONBLOCK
+/// will fail with ENXIO if there is no reader already attached to it.
+/// NOTE: writing to a pipe will start failing when reaching 64K of unconsumed content.
+pub fn open_file_nonblock(path: &Path) -> Result<File, std::io::Error> {
     OpenOptions::new()
         .custom_flags(O_NONBLOCK)
         .create(true)
+        .read(true)
         .write(true)
         .open(path)
 }

src/vmm/src/vmm_config/metrics.rs

@@ -7,7 +7,7 @@ use std::path::PathBuf;
 use serde::{Deserialize, Serialize};
 
 use crate::logger::{FcLineWriter, METRICS};
-use crate::utils::open_file_write_nonblock;
+use crate::utils::open_file_nonblock;
 
 /// Strongly typed structure used to describe the metrics system.
 #[derive(Clone, Debug, PartialEq, Eq, Deserialize, Serialize)]
@@ -26,7 +26,7 @@ pub enum MetricsConfigError {
 /// Configures the metrics as described in `metrics_cfg`.
 pub fn init_metrics(metrics_cfg: MetricsConfig) -> Result<(), MetricsConfigError> {
     let writer = FcLineWriter::new(
-        open_file_write_nonblock(&metrics_cfg.metrics_path)
+        open_file_nonblock(&metrics_cfg.metrics_path)
             .map_err(|err| MetricsConfigError::InitializationFailure(err.to_string()))?,
     );
     METRICS