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Date: Tue, 18 Feb 2025 12:03:42 +0100
From: Wolfgang Bumiller <w.bumiller@proxmox.com>
To: Fiona Ebner <f.ebner@proxmox.com>
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Subject: Re: [pve-devel] [PATCH qemu-server v3 2/3] config: add special
class that prevents writing the configuration
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On Wed, Feb 12, 2025 at 12:04:26PM +0100, Fiona Ebner wrote:
> To be used in the context of template backup, where a minimized
> temporary configuration is created to start the VM in 'prelaunch'
> mode. Issue a warning, so that code paths where this happens will be
> noted and can be evaluated and adapted.
> =
> Since the code currently doesn't use blessed config objects, special
> dispatching is needed to potentially defer to the new child class in
> the write_config() method.
> =
> Suggested-by: Fabian Gr=FCnbichler <f.gruenbichler@proxmox.com>
> Suggested-by: Thomas Lamprecht <t.lamprecht@proxmox.com>
> Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>
> ---
> =
> Changes in v3:
> * use dedicated class
> =
> PVE/Makefile | 1 +
> PVE/QemuConfig.pm | 15 +++++++++++++++
> PVE/QemuConfig/Makefile | 5 +++++
> PVE/QemuConfig/NoWrite.pm | 25 +++++++++++++++++++++++++
> 4 files changed, 46 insertions(+)
> create mode 100644 PVE/QemuConfig/Makefile
> create mode 100644 PVE/QemuConfig/NoWrite.pm
> =
> diff --git a/PVE/Makefile b/PVE/Makefile
> index dc173681..440fbe77 100644
> --- a/PVE/Makefile
> +++ b/PVE/Makefile
> @@ -11,4 +11,5 @@ install:
> $(MAKE) -C VZDump install
> $(MAKE) -C API2 install
> $(MAKE) -C CLI install
> + $(MAKE) -C QemuConfig install
> $(MAKE) -C QemuServer install
> diff --git a/PVE/QemuConfig.pm b/PVE/QemuConfig.pm
> index ffdf9f03..b60cc398 100644
> --- a/PVE/QemuConfig.pm
> +++ b/PVE/QemuConfig.pm
> @@ -3,6 +3,8 @@ package PVE::QemuConfig;
> use strict;
> use warnings;
> =
> +use Scalar::Util qw(blessed);
> +
> use PVE::AbstractConfig;
> use PVE::INotify;
> use PVE::JSONSchema;
> @@ -561,6 +563,19 @@ sub get_derived_property {
> }
> }
> =
> +sub write_config {
> + my ($class, $vmid, $conf) =3D @_;
> +
> + # Dispatch to class the object was blessed with if caller invoked th=
e method via the
> + # 'PVE::QemuConfig' class name explicitly. This is hack, but the cod=
e currently doesn't
> + # generally use blessed config objects. Safeguard against infinite r=
ecursion.
> + if (blessed($conf) && !blessed($class)) {
> + return $conf->write_config($vmid, $conf);
> + }
> +
> + return $class->SUPER::write_config($vmid, $conf);
> +}
> +
> # END implemented abstract methods from PVE::AbstractConfig
> =
> sub has_cloudinit {
> diff --git a/PVE/QemuConfig/Makefile b/PVE/QemuConfig/Makefile
> new file mode 100644
> index 00000000..c3a0d277
> --- /dev/null
> +++ b/PVE/QemuConfig/Makefile
> @@ -0,0 +1,5 @@
> +SOURCES=3DNoWrite.pm
> +
> +.PHONY: install
> +install: ${SOURCES}
> + for i in ${SOURCES}; do install -D -m 0644 $$i ${DESTDIR}${PERLDIR}/PVE=
/QemuConfig/$$i; done
> diff --git a/PVE/QemuConfig/NoWrite.pm b/PVE/QemuConfig/NoWrite.pm
> new file mode 100644
> index 00000000..f697506f
> --- /dev/null
> +++ b/PVE/QemuConfig/NoWrite.pm
> @@ -0,0 +1,25 @@
> +package PVE::QemuConfig::NoWrite;
> +
> +use strict;
> +use warnings;
> +
> +use PVE::RESTEnvironment qw(log_warn);
> +
> +use base qw(PVE::QemuConfig);
> +
> +sub new {
> + my ($class, $conf) =3D @_;
> +
> + bless($conf, $class);
^ This blesses the original reference.
In the next patch it is used as:
$newconf =3D NoWrite->new($newconf);
which *suggests* that the original stays untouched - which is not the
case.
Generally, I'd recommend such methods to do a shallow copy for the sake
of clarity (simply start with `$conf =3D {%$conf};`).
Otherwise the following code counter-intuitively prints
'PVE::QemuConfig::NoWrite'.
my $original =3D { ... };
my $blessed =3D NoWrite->new($original);
print(ref($original));
> +
> + return $conf;
> +}
> +
> +sub write_config {
> + my ($class, $vmid, $conf) =3D @_;
> +
> + log_warn("refusing to write temporary configuration");
> + return;
> +}
> +
> +1;
> -- =
> 2.39.5
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